## How To Determine the Correct Size Of Tires For Your Imperial

How To Calculate The Correct Tire Dimensions For Your Imperial:

The first number is the width of the tire in millimeters, measured from sidewall to sidewall. To convert to inches, divide by 25.4 In the example above, the width is 185mm or 7.28".

The second number is the aspect ratio. This is a ratio of sidewall height (also referred to as "section height" in the diagram) to width. In the example above, the tire is 7.28" wide, multiply that by the aspect ratio to find the height of one sidewall. In this case, 185x0.60=111mm or 7.28"x0.60=4.36".

The last number is the diameter of the wheel in inches.

To figure the outside diameter of a tire, take the sidewall height and multiply by 2, (remember that the diameter is made up of 2 sidewalls, the one above the wheel, and the one below the wheel) and add the diameter of the wheel to get your answer.

Example...185/60R14 85H or 185/60HR14

185mm x .60=111mm x 2=222mm + 355.6mm(14")= 577.6mm or 22.74"

Tips from Dick on How To Calculate Your Tire Size:

Here is how you figure out the dimensions on a tire and what will work for your car. The first number is the width in Millimeters "255" is 255 mm, or about 10". I don't know what year your car is, but chances are these tires will be at least 1" and maybe almost 2" wider. You'll have to look to see if you have clearance, between the fender skirt and the rear wheel, and between the frame and the inside of the front tire in a sharp turn. Remember when you hit a bump in a turn, the tire will move a little closer to the frame.

The second number is the percentage relation between the height from the rim to the road as compared to the width. "70" means the tire is 70% as high as it is wide, or 7 inches from rim to tread (with no load on the tire). This will be a short, fat looking tire for your car, when the original was a tall thin dude. Some like this new look, some don't. You pick it.

Tips from Chris on Tire Measurement:

The measurement of the sidewall height on tires is always done as a function (or percentage) of the section width, and it's called the "aspect ratio."

In the 1950s and early 1960s, the standard aspect ratio was 82-84%, meaning the distance from the top edge of the rim to the top of the tire tread was that percentage of the width of the tire (which is measured about mid-carcass, not at the tread). Tire sizes were usually given as simply the section width and rim size, like 9.15-15 (9.15 inches wide, 15-inch rim).

Around 1970, bias-ply tires moved to a 78% aspect ratio, this the term "H78-15. H was a load rating, 78 was the aspect ratio, 15 the rim size. All tires using the same load rating (for example, G70-15, G60-15, etc.) were the same diameter. Because the diameter was the same and the aspect ratio was lower, a 60-series tire was wider than a 78-series.

As radials became the norm, these designations stayed in use, but an R (for radial, of course) was added: HR78-15. In the 1970s, US tire manufacturers adopted the European system of metric sizing (though the rim diameter is still in inches). Example: 235/75R15. R still stands for Radial, 235mm is the section width, 75 the aspect ratio, 15 the rim diameter. What's different in this system is that the section width is an absolute number, so a 235/75 is just as wide as a 235/60. The 60-series tire is therefore smaller in diameter.

During this time period, the "standard" aspect ratio dropped to 75% (today, 65%, 60% and even down to 35% on some exotic sports cars, are all common).

So, if you wanted to put a set of radial tires on an old car and approximate the original sidewall height, you'd have to go with an enormously wide tire, which is undesirable for many reasons, including more difficult steering and some odd handling response (for these cars, which were not designed to have such a big contact patch between rubber and road).

As such, we compromise with a balance of width and height that comes closest overall... an old 9.15-15 might become a slightly wider but shorter HR78-15, which then becomes an even shorter 235/75R15.

Finding correct-size 14-inch tires today is harder still, because few tire manufacturers make little 14's strong enough and wide enough to manage the load of a 5,000-lb car. Choose carefully, note the load rating (remember, some 65% of your car's weight is on the front wheels) and the temperature grade (B is OK, A is better, C is not good enough), and maintain proper tire pressure (about 4 lbs below max is what I use). We've all seen what happens when under-inflated tires overheat in a heavy vehicle like, oh, I dunno... a Ford Explorer?

Another tip from Dick:

The bottom line for safety is to note the weight of your car, figure that maybe as much as 60% of that could be on the front wheels (yes, I know it's not that bad, but we're being conservative here), divide that by two and you will get a static load on your front wheels of perhaps 1500 pounds per wheel. Any tire you buy will have a load rating on the sidewall: compare that with the weight just calculated and make sure you have a safety factor, around 25 to 30 % is adequate. This means you need a tire which can carry almost 2000 pounds.

Question from Nik (1957):

What size tires were original to the '57? I see pictures of these cars with the ultra-wide whitewalls and I think they look great, but who makes them?

Replies:

From Alex:

The '57 came with 950 +14 tires. A radial equal to this size would be a 235+14 which I have yet to find.  The white wall should be 2 1/4 wide. The first set I bought came from Lucas, all four tires came apart. I then bought tires from Coker, I got the B F Goodyear type.  Big, big difference.

From Jeff:

True, and available exactly as original from Coker. Whitewalls, as we have discussed in great detail, trended narrower after 1956, and the 2 1/4 is exactly correct for your car.

There is no acceptable radial tire that I am aware off. This tire has an extremely tall sidewall, there is no radial equivalent that I have seen. This means that if you go with radials, you will also have to switch to 15" wheels (as Imperial did mid-year in 1959), or you will lose 2-3 inches of ground clearance, and render the car incapable of even the gentlest sloping driveway. Watch out, exhaust system......

Question from Mel (1959):

I note that one of the tire sizes offered for 1959 was 9.50 X 14. This omits the second number which is the height of the tire as a percentage of the width. There must have been a standard at that time. I recall something about the number 78 but??? Would someone clarify this for me please.

Replies:

From Neal:

While I'm certainly no expert on tires, the 9.50 X 14 size was used on the old bias ply tires. 14 was the size of the wheel diameter, and sometime in '59 the Imperial went to 15" wheels. I'm guessing that the 9.50 refers to the width of the tire.

Now the tires are sized like 225-75R15, where, working backwards, 15 is the radius of the tire, R means radial, 75 means the ratio of height of tire from road to rim/width of tire; and 225 is the width of the tire (in millimeters?). On new cars that are designed for handling rather than comfort (and what a misguided notion THAT was!), the "75" ratio would be lower. For example, my '97 Riviera has 225/60R16 tires (225 millimeters wide, 16" diameter wheels, and the height of the sidewall is 60% of 225, or 135 millimeters. Believe me, it rides much more harshly than my older cars.

Back in the 1970s, the tires sizes changed designations. I have a 1977 Chrysler owners manual I salvaged out of my long-gone '77 Cordoba, which lists various wheels and tires that could have been put on the cars. GR78-15 and GR70-15 ("R" meant it was a radial; the absence of "R" meant it wasn't a radial, duh!) were okay on the Cordoba, and HR78-15, JR78-15 and LR78-15 were fine for the NYB and Newport. The 15 would again designate the wheel size; and I'm supposing the H, J or L would be the equivalent of the current "225" and the "78" would be the ratio.

From Philippe:

9.50 x 14 tires are between "80 and 85 series", this number is the ratio width/height of tire side : look at a tire catalog (like the online Coker catalog) and search the 9.50x14 (US Royal): you'll see that this tire has a 29.50 diameter. You calculate 29.50 - 14 (diameter of wheel) = 15.50 / 2 = 7.75 which is the tire side height. Now 7.75 / 9.50 x 100 = 80 % ratio. Seems that the real width (cross section) of the tire is 9.05 and not 9.50 so it would be a 85 series.

If you do the same arithmetic with a L78x15 you find around 80. And with the 235/75R14 you find .. 75 !

There's also at the "WWW radial" page a cross reference table between bias, radial etc.. Coker names the bias series like 9.50 x 14 the "83 series" (just between 80 and 85..)

Question from John (1959):

Does anyone know the correct size of tire for a '59 LeBaron. They are 15"s and that's about all I know. -- I'd like to put radials on it....
Replies:

From Jeff I:

I bought a new set or R235-75-15s on mine a couple of months ago. That is what they said is close. Boy did they improve my ride and handling over the old bias plys I had on it!

From Jeff S:

To be totally honest, there isn't a modern equivalent that is appropriate. Your car had an extremely tall and narrow tire, a very high bias as they say. Modern radials are comparatively much wider and the sidewall is not nearly as tall. Thus, placing a modern radial on your 1957 will drop the ground clearance by at least 2 inches. If you must run radials, maybe should consider the 15 wheels from a mid 59 or newer with P235/75-R15's. That will pretty closely approximate the correct ride height.

From Philippe:

I've the same problem with my 57 ! I've two solutions: - Keep the original size, drive with bias and have a tall car at the rear (and perhaps problem to put them in the rear wheel well). - Buy 225 R 75 - 14 radials (Coker): the car would be lower, but on radials ! This radial have 27.62" diameter and 6.04" tread size. The 9.50 x 14 Lester is taller (29.75) but tread is 5.75" . So the car would be 1 " lower. When I bought my car, it was fitted with (old) 215R75 x 14 and it looked fine with them. The other solution is to find Imperial 15" wheels (55/56 or 59..61) and put the 235R75 x 15. But you need also the 15" wheel-covers!

From Matt:

The closest size I found to this size (same as a '65) is a radial 235R P75/15. Mine from Shamrock Tires \$100 ea.

From Denis:

Beginning of the model run size was 9.50 x 14. End of the model run 8.90 x 15, just another of those splits car mfg's were noted for.

Question from Timothy (1960):

Can anyone tell me what the correct size of tires I need for a '60 Imperial.

Replies:

From John:

P235 is the correct size.

You can do the math for your tire size by multiplying your tire size by 25.5. Its a rough conversion but pretty darn close as tire sizes do not match up exactly. For your tire size, multiply 8.20 X 25.5 = 209.1. You could probably be comfortable with 215/75R-15's. 205's would be too small, 225's may be too big.  My '66 calls for 9.15 -15's so my calculation worked out to just over 233. I put on 235's and am well pleased with how they fill up the wheel well.

Follow-up from Chris:

I would disagree, and suggest that the largest tire that can be bought be put under a 5200 lb. Imperial. A 75 series 235 x15 radial would be the minimum. I have had both the original 9:15's and R235's on my '66 Imperial. The 9:15's have nearly 2 inches more in diameter than the 235's, lifting the car off the ground at least an inch higher. In comparison, the 235's look puny, but I've gotten used to the appearance - a trade-off for better handling and availability.  To my eye, the 2:15's would make a '60 Imperial look like it was on roller skates, and possibly wouldn't have sufficient load rating.

From Marcus:

General makes a 255 70R 15 which looks much better than the 235.

From Leo:

For Goodyear Collector tires with the original tread design and whitewall width, contact Jim Benjaminson, (Plymouth Owners Club Treasurer) for prices and availability. He can be reached at 701-549-3726

From Chris:

When I bought my '67 in 1989 the original owner had somehow been talked into putting 255s on the car. They looked like tires from a truck or motor-home... simply too large, and that much tread on a car designed for much narrower 9.15x15 bias-ply tires puts too much stress on the steering gear and overall suspension geometry. I switched them to a more compatible 235/75R15 XL (Extra Load) Firestone 721 with an 1-5/8" whitewall soon after and have been happy with the ride, handling and performance since.

From Norm:

As I am a stickler for perfect performance before perfect appearance, my question is: do the Firestone 721 tires balance perfectly so as to give a totally (tire) vibration free ride? I have used Michelins exclusively for this reason only but would like the added benefit of a wider whitewall provided the tires do not shake-at all and at any speed. As for 255x70 15's I cannot imagine them being balanceable to zero shake as there is simply too much mass to compensate for and General has never been noted for its lack of runout or exceptional quality control. PLEASE correct me if my impressions are incorrect and they are balanceable and vibration free.

From Dave:

Our '60 is now fitted with L78 x 15 Cokers. It looks better and I think handles better.

Question from Bill (1964):

Can anyone tell me what type and size tires may have come (factory) on my '64 Crown 4-door?

Bias Ply 8.20 x 15. I always believed they were undersized and when we had need to replace the tires on our then-2 year old '65, we went with 9.15x15.After that, it was glass belted L 78 x 15's . Today it would be 235 75 R 15. If you put old, bias ply tires back on your car, be prepared for squirrelly handling.

Tips from Bill (1968):

Tires for a 1968 Imperial would have been 9:15 x 15 from the factory. Chrysler used quite a few brands, as you mentioned. I would think they would have been steel belted. Blackwalls were standard on the Crowns with whitewalls, as an option. If I recall, I think many of the large bodies that were built by Chrysler and Imperial had Uniroyal Tiger Paws and Good year tires as the most popular. It was quite common for dealers to take off the standard tires and put on a different brand or style that they were connected with. Your companies like Coker tire offer quality reproductions of the originals. Keep in mind that tires were not made in 1968 like they are today. Similar to computers, TV's, etc. there has been drastic improvements. The radial being just one. Hogwash to the radial tuned suspension, sounds like a GM advertisement. Granted suspensions have also been refined and improved but the torsion bar suspension was the best available and way ahead of its time! If you are not trying to show your car and achieve a perfect 100 point concours restoration, go with radials. You will notice extremely better ride and handling.

Question from Jack (1968):

I'm getting ready to buy tires for my 68 Crown. I know a couple months ago that all were talking about what good tires to buy. I went to BJ's and found Michelin XW4 to be \$95. installed. This is \$20.00 better than Sears.  But that's a lot of money for 5 tires for on a car that I might drive 3000 miles a year. They will dry rot before I show any wear. There are a lot of tires that are \$60. to \$70. that I think will do. BJ's also has B.F.Goodrich and Uniroyal all for a lot less, I can save \$120.00 by going this way. I don't care for Uniroyal much, but would like to know if anyone has the Goodrich's, and how do you like them? Any other input would be good also. Montgomery Wards had General's on sale for \$50.00 too. I plan to use P235/70R15, is this good?

Replies:

From Lawrence:

I just a few thoughts about tires- off-brand tires are made (for the most part) by name brand manufacturers- look at the side wall - there is a set of numbers and letters on all tires that are indicators of quality and service life- traction, wear index #(the higher the better) load range, temperature. 70 series tires are shorter than 75 series tires for the same given width (aspect ratio) look on your drivers door post- there should be a sticker which indicates the proper size of the original tires- your tire dealer will have a chart- which converts the "68 size to modern size. if you use a tire taller or shorter than stock it may upset your speedo calibration. Finally- if you drive your car infrequently you may wish to invest in a set of tire covers to protect your tires from the ravages of solar radiation and uv rays.

From Gregg:

I'd just like to offer my two cents, if I may... I use that size, too. I use them on my 78 Thunderbird and if I buy a Chrysler product I will use them on that, too. My first set of 235/70/15's were Goodrich, but Goodrich doesn't make that size in a whitewall anymore. Luckily, I found out that Pep Boys has them in stock for about 50 Bucks, same price as the others that you mentioned. Now, I use 70 series tires because my car came with them from the factory. When I bought the car, it had an original Hr70-15 Goodyear custom Steelguard in the trunk. However, my car also came with 6.5" rims. Many Chrysler Co products came with skinny 5.5" rims. What is the width of the rims on a 68 Imperial? An inch or a half inch makes a lot of difference. I think you would need at least 6" rims for a 70 series tire. Otherwise the tires will wear funny. Also, the 50 dollar tires will dry-rot faster that Michelins. Michelins are a great tire. But they're not necessarily worth \$105 a tire!! The biggest draw for me for buying the Michelin XW4's is that they have a narrow band white wall like the tires of the late 60's and early seventies. My Goodyear was a narrow whitewall. A narrow band whitewall might make the car look more authentic. I'm not that dedicated to authenticity, so my cars will have common white stripe tires. BTW, I think modern tires dry-rot much faster than they used to. I have a 1976 Uniroyal that is still usable, but my new tires only last three years or so before the rubber starts separating. More planned obsolescence?

From Dick:

I think the extra money for the Michelins is well worth it! That is exactly what I have on my 68, and after 12 years they are still perfect in all respects, no sign of age cracking, smooth and quiet. The 235R70's are going to look rather low and wide on that car, I think 235R75's are a better choice (almost an inch taller, same width).

From Elijah:

I'll second Dick on this one -- even if you don't go with the Michelin's, do get 235 R75s. And you really WILL be amazed at the difference the Michelins will make. When I first got my '71, we put a new set of Goodyear Tiempos on the car -- and I can't tell you what a huge mistake that was. Even aside from the fact that the weight of the car ate off the tread within 15,000 miles (on a 50,000 mile tire), these tires gave the car terrible handling.

The set of Michelin's I have now has gone over five years and 60,000 miles, and are still in terrific condition. But the best part is the improvement in drivability -- truly an Imperial ride, and, in my mind, well worth the extra expense.

From Mark:

I agree, Dick. The thinking in tires has changed since 1968. In '68, the look was "tall and narrow" (relatively narrow). The R75s seem better for the '68.

I also try to find a tire that has a distinct "shoulder" to it - don't know if that's the right term. If you cut it in half, it has a sharper "edges" on it - a flat top with a pronounced drop-off toward the sidewalls. This, to me, approximates the look of the bias-belted tires that came with the car and keep it looking "correct." Modern radials are far more rounded in appearance, with lower sidewalls and don't always look right on older cars.

I also agree with you, tires are one place not to skimp. Even if you only put low mileage on your cars, they're worth it for the peace of mind.

But I would still feel okay recommending the Arizonians, though I'm sure they're probably not as good as Michelins.

Question from Phil (1968):

The owner's manual for my '68 Crown calls for a 9.15-15 tires. Is this the same as a 235 75R15 or just what equates to the original and correct size in today's tire terminology?

Replies:

From Mike:

I use 235/75 R15 on my '67...I suppose you could use 235/70 R15 which would give a slightly shorter sidewall (and lower speedometer reading), though it wouldn't fill up the wheel wells as much. Interestingly, I found that my 235/75/R15 seem to make my speedo read slightly less (about 1 mph for every 30 I'm going) than it should. I also found it was hard finding tires with white walls in low-profile designs. At some point the whole automotive world went from whitewalls and hubcaps to low profile blackwalls and aluminum rims.

Michelins are great tires...my tires are actually the non-name brand Michelins, Ameritrak or something...they're made by Michelin, but only cost me 49 bucks each installed. They ride very nice. When I get the car back from the garage (today they said they got the master cylinder on, but the rear brakes don't work and the emergency cable is still stuck) I'll check and see the name on the tires.

From Bob:

The 235/75R15 is a modern size radial, OK for an Imperial, with a slightly smaller diameter than the 9.15-15, I believe. There are formulas at www.tirerack.com that will give exact diameters and let you calculate possible speedo error.

From Steve:

I'm using 235 75R15 on my 64 Crown Coupe and they work and look just fine. Wish they had wide whitewalls, though.

From Ross:

Firestone's FR 721 has a nice 1 1/2" whitewall, and they give a great ride. I have them on my '78 New Yorker Brougham, and the car looks 'factory'.

From Chris:

I have the FR721s on both my '78 NYB and my '67 Crown. The whitewall in the 235/75R15 size is actually 1-5/8" and is an exact match for the factory-optional "wider whitewall" tires offered by Chrysler on the NYB when it was new (I actually have two of the original tires, though not on the car anymore due solely to their age).

The rolling radius of the 235/75R15 is not quite as large as the original 9.15x15 tires that came on my '67 and your '68, but it's as close as you'll get in a modern tire. The new tire is also somewhat wider. The original tires were either an 82% or 78% aspect ratio (I never have been able to confirm which), while the new tire is a 75% ratio (meaning the sidewall height is 75% of the section width of the tire). I have not noticed any significant speedometer error in either car.

By the way, the FR721 is also XL (extra load) rated, important when mounting tires on a 5,000-lb car!

And despite the bad rap Firestone's getting lately, I personally have had no problems with these tires in over 8 years of driving. I'm no Firestone loyalist (don't have them on any other of my cars), but these look great and work well.

Question from Paul (1972):

Please tell me what size tire I should be looking for. I own a 1972 imperial with L 84 / 15 size tires. I looked in the owners manual and it listed these as stock. My car only has 2400 original miles so I'm sure it is what came on it.

Replies:

From Elijah:

On both of my '71 Imperials, we have always used P235 75R 15 inch tires, preferably Michelin radials. I have found that the Michelins provide superior performance, handling qualities, and tread wear life for the Imperial.

These Michelins are whitewalls. Someone a while back inquired about the width of the whitewalls, and I haven't had a chance to measure them yet. The original tires had a two-ring whitewall (large inner, small outer).

From Dick:

I don't think you will find the exact match, but I would recommend 235R15's for your car, and I think you would be most pleased with Michelins. Even though they cost quite a bit more than everyday brands, they ride smoother and quieter, handle better, and last forever! I have them on my 68 Crown and my '81, also on my truck and my 56 Packard, I am very pleased at the improvement in the way the cars feel with these tires. If you have the Price club/Costco stores in your area, they are the place to buy Michelins!

From ??:

Michelin makes an excellent tire that would be good for your 72 Imperial. The model of Michelin is "XH" which is an extremely durable, long lasting tire. They do make the 235R X 75 15 inch tire which is comparable to L78 x 15. I have purchased this make of tire and found it to be long lasting and very quiet. The tread pattern will not have the problem of catching the grooves in pavement that other tires do.

From Dale:

Make sure you specify "extra load" tires in whatever brand you get. many will have it on the tire size. i.e. P235/75r15XL

Question from Abe (1981-1983):

I recently purchased an '82 FS Imperial...the tires are oversized (225-75's!!!).

Thus, I will be buying new tires...The "book" calls for 205-75's, I was considering the following: 1. 215-70's, or 2. 225-65's, or 3. 215-70's for the front and 225-65's on the rear.

Any opinions? Will the 225-65's be too wide for the front or rear fender walls? I just think the 205-75's look too small!!! I was even considering 225-70's!!!

Replies:

From Bob:

Abe, I put 225/70s on all of my 81s. Absolutely no clearance problems at all.

From Bill:

I have 225R70's on my 81. The Pep Boys Furtura 2000 is a nice tire but you can put Michelin MX4 RainForce on for less money if you shop. When you shop at Pep Boys for tires at least in NY you need to add in mounting, balance, valves and old tire disposal fee's, with other tire houses like Mavis and WTC you don't and the better tire becomes cheaper.

From Scott:

I don't know if you have a Cooper Tire dealer in your area, but I used to get Cooper tires with the wider whitewall when I had my '82 Cordoba and my mom and dad have Coopers on their '81 Mirada........

From Dick:

The correct size is 205/75R15, not 215..... The difference? 10MM in width, and about 8 MM in ride height, which will make the car accelerate somewhat slower, the speedometer and odometer read low, and the fuel economy seem poorer, although it will actually be better (your "miles" are longer), all by about 2%. The width difference could cause geometry and interference problems in a sharp turn combined with a bump. They are also more expensive by a small amount. The original tires had a whitewall width of about 1 1/4 inches, which are usually not too hard to find, even in a regular folks type store. Michelin was available from the factory, along with other brands (11.1% of '81 had Michelin).

Question from David (1981-1983):

What's the regular size tire for this car? My 81 has the 225-70R-15's?

Replies:

From Bob:

There were two tires available for the '81-'83 Imperials: Goodyear and Michelin. The whitewalls of both were the same width - 32 millimeters, which is only slightly more than 1 1/4".

From Dick:

Your tire size is posted on the driver's side "B" post, but to save you the trouble, it is 205-70R15. Your tires are two sizes (20mm = almost an inch) too big, I'm surprised that you don't have clearance problems.

From Rob:

I have been assured that 275 60-15 will fit in the rear with almost no clearance problems and that 255 60-15 will fit in the front. I haven't put them on mine yet. I am waiting for the 215 70 15 Dunlops (no clearance problems) to wear out. They have like a 500 tread wear rating. It is taking forever.

Follow-up from Chris:

I don't know who assured you of this, but even if such enormous tires would fit, the car will look (and handle) like a dragster. This is way too much width (contact patch) for the steering and suspension, and way too much height difference for the speedometer. The rear tires are a good 15% larger in diameter than the stock 205/70-15s. And the tires would be far too wide to safely mount on your existing wheels (they're almost 3 inches wider!).

A 235/60-15 tire is a pretty close match for rolling diameter, and that is as large as I would go. If there are no clearance problems, the extra inch or so of tread width they'll give should be managed OK by the steering and suspension. A 215/65-15, if you could fine one, would work as well.