235/85 R15 (Doesn't Exist, I Know)

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bajajoaquin

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Looking through the discussions about the proper tire size for our cars, the consensus is that it's 235/75 R 15. It seems that the consensus is because there is no actual modern equivalent to the original fitment bias ply tire (9.15-15, right?)

At 9.3" wide, the width is about right, but bias tires had about an 82% aspect ratio, meaning the overall height was about 30.5". The 235/75 is just under 29.

It seems to me that the proper size would be 235/85 R15. It would be the right height and width. I know that radials have softer sidewalls, which is one reason for lower aspect ratios, but they make 85-series truck tires, so it's not like this is an impossible design.

Coker makes all sorts of low-volume tires. I would have thought that they would have made a tire in this size by now. It's not just C-bodies, but I'd imagine GM and FoMoCo full-size cars had a similar tire size.

Anyone know why there's no tire available in this size?
 
An 85-series truck tire would be pretty rough-riding, especially since you see these in a 16 and 17-inch size on one-ton pickups, and are available in 8-ply through 20-ply sizes. You'd want a 78-series tire, which was normal in the '70s. Coker and such would be the source for those, but they are going to be expensive.
 
That doesn't really answer the question, since I'm not trying to buy truck tires, and Coker doesn't sell them. I'm also asking about an aspect ratio that was normal in the 60s: 82%-83%.

I understand that they're not available anywhere. I'm just curious why not. There is a big enough market to produce bias ply tires for 50s cars. That can't be a huge market. Why not correctly-sized radial tires for 60s cars?
 
Because an 85-series tire was never considered necessary in a 14- or a 15-inch size. An 85-series tire has always been a truck tire for as long as they've been around. As I said before, the 78-series tire was the norm for cars in the '70s, along with 70, 60, and 50 for many years. Remember, that numbering series didn't exist in the '60s, anyway, at least in the US.

When they changed the system from say 7.75-15 to H78-15 then HR78-15, in the early '70s, then the metric way later on, to 235-75R15. So, the correct size for '60s and '70s cars already exists. If you were to put an 85-series tire on, you'd have a goofy-looking, very skinny, and very tall tire.
 
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I really don't understand where you're coming from.

The factory original tire came out before the '70s. In the 50s and 60s, the normal aspect ratio was 82%-83%. So a 9.15-15 tire (the factory original size on my car) was about 30.2 inches tall. That may be considered goofy-looking and skinny to you, but if I bought bias ply tires in that size, that's what they would be.

Everyone has gone to 235/75-R15, including me. It's as close as is currently available. I'm not disputing that.

I'm not asking why all 85-series tires are truck tires. I'm asking why nobody makes a car tire with that aspect ratio for a market that seems to be at least as big as the market for 9.5-14 bias tires. Someone makes those.
 
does the overall radial height take into account the squat when loaded? it seems you could lose about 1/2 inch in height with the vehicle weight.
 
OK, I understand 100% about what you are asking, and have from the get-go. You want a new, tall, skinny tire. About the only thing you are going to find in a 15" tire is a 7.50-15 8-ply bias trailer tire. I have these on my car hauler. They'll last forever. You'll have a rough ride, but you'll have your 85-series aspect ratio. Go to any trailer store and you will see what I mean. They are out there. They make a 800-15 and a 875-15 trailer tire, too.
 
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I'm not sure 9.15x15 were used on your car but this is on the data plate.

Here is some info on the tires for my car for reference.
The original tires were 8.55x15 and per Goodyear (the company not the local tire store) they are the exact tire that was later a H-78.
The closest tire to that today is a 235/75

Like mentioned the 78 series tire replaced the "square" tires when they started selling low profile tires (70 series)

Alan
 
This is what he's asking "Anyone know why there's no tire available in this size? " and he still hasnt gotten the answer, from what I see. O.K. there is a truck tire, o.k. there is a trailer tire but why not a car tire? Is that what you're asking?

For the record, I have no idea what tires equal what size.........
 
I haven't checked my data plate, but I have checked my owner's manual, and 9.15-15 is the OE fitment (just checked FSM, which agrees.) I sent an email to Coker to ask them, and I'll probably just hear that the market isn't big enough. But Cadillac used 9.00-15 in '66 through 69. Lincoln used 9.15-15 in at least 64-68. I would think just those would be as big a market as for the people who are buying 9.5-14 tires out there.

As for what tire sizes "equal" my understanding, from information in the Online Imperial Club archives, is that there are three main generations of tire size nomenclature. The bias all-numeric, the bias alpha-numeric, and the metric (now associated with radials).

The all-numeric sizing went until about 1970. These tires had sizes like 9.15-15, 8.00-14, etc. There's some extra detail about decimals and whether it ended in .50, and I'm not at all clear on that. But the basic passenger car tires were between 82% and 84% aspect ratio. So a 9.15-15 tire had a 7.5" sidewall.

In about 1970, they changed to alpha-numeric tires, and I'm really not super clear on this either. I think that aspect ratios in the mid-70% range is a good rule of thumb, but I think there was a system where the letter related to load range, and load ranges all had a certain height, so if it got wider with a certain load range, the tire got lower aspect ratio. Or something like that. I'm sure it made sense to people in the industry, but seems pretty jargon-filled and silly to me.

Last (and current) is the metric system we use today. My own opinion is that it's the best system, since you can tell all the info just by what is on the sidewall, and you don't need to have any inside information. P235/75 R15 means Passenger car tire, 235mm wide, sidewalls are 75% of the width, 15" wheel.

I hope that's what you meant when you said you had no idea what tires equal what size, otherwise I just wrote all that for nothing....
 
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polara71 said:
This is what he's asking "Anyone know why there's no tire available in this size? " and he still hasnt gotten the answer, from what I see. O.K. there is a truck tire, o.k. there is a trailer tire but why not a car tire? Is that what you're asking?

For the record, I have no idea what tires equal what size.........
Click to expand...

To answer the question. Supply and demand. There aren't enough people demanding a 9.15 x 15 bias ply tire over 30 inches tall. In fact the tires we grew up with in the 60's, 70's and 80's with 78 series profiles are falling out of favor due to lack of demand.
Modern cars have larger disk brakes, which need bigger vented wheels to prevent overheating. So, 16-22 inch wheels with low profile tires are the norm. The funny thing is the smallest cars have 14-15 inch wheels with low profile rubber and I've seen people putting those 40-50 series profiles on older cars because they are the cheapest available. Old cars look like they're running around on space saver tires.
Coker is the only one with something close. It's 900 x 15 at 29.59 inches tall. The only radial close to that is the aforementioned 235/75 R15. Visually it's hard to see a 1/2 inch difference in the height of those two bias ply tires. The Radial will look shorter and fatter, which it is, but ride so much nicer.
As to the overall picture of this post, every manufacture of a tire has their own molds for a given tire size. Given that, there will be some tolerance. An equally marked tire size from Goodyear is not going to be the same as a Firestone. Kind of like clothing. This and the rubber type, tread pattern and overall construction is why manufactures say don't mix and match tires.
I'm not an expert, but my dad sold tires after his career in the military for around 10 years and I would help out in the summer.
 
Outside of Coker et al. I don't believe I've seen an 85 series made in years. Who are you referring to?

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