C3 tire size

[Jim Callan]

What is an appropiate tire size? 225/70 or 235/70. Will a 235/70 fit in the spare tire carrier ?

Thanks

[Joe L.]

Re: C3 tire size

Jim-----

The closest size to original is the 215-70-15. However, there are very few tire choices out there anymore in this size. The 225-70-15 will also work and is close to the original size in most respects. There are a few good choices still out there in this size. Either of these sizes will fit in the spare tire tray, but a different front bolt length may be required.

The 235-70-15 is too large, in my opinion, for use on many C3s, especially the 68-69 variety. Using this size, there may or may not be rubbing problems under certain conditions. I don't know if this size will fit in the spare tire tub, or not. The 235-70-15 is almost 1" greater in OD than the original F-70-15 tires used on 68-72 C3s.

[Duke W.]

Re: C3 tire size

What year? OE wheels?

Duke

[Mike Cobine]

Maybe a 225-70 or a 235-60 instead

I hate to disagree, but I think the tire sizes ran different.

6.00 .. A78 .. A70 .. A60 .. 185-60 .. 175-70 .. 165-78
6.50 .. B78 .. B70 .. B60 .. 195-60 .. 185-70 .. 175-78
6.70 .. C78 .. C70 .. C60 .. 205-60 .. 195-70 .. 185-78
7.00 .. D78 .. D70 .. D60 .. 215-60 .. 205-70 .. 195-78
7.35 .. E78 .. E70 .. E60 .. 225-60 .. 215-70 .. 205-78
7.75 .. F78 .. F70 .. F60 .. 235-60 .. 225-70 .. 215-78
8.25 .. G78 .. G70 .. G60 .. 245-60 .. 235-70 .. 225-78
8.55 .. H78 .. H70 .. H60 .. 255-60 .. 245-70 .. 235-78

The GR70-15 was the standard tire in 1973, increasing from the F70 in 1972. It later became metric at 235-70R-15. So the 225-70R-15 would be the equivalent to the F70, not the 215-70R-15.

215 mm sectional width is about 8.4 inches. On an 8 inch rim, that would be the minimum you'd want to run. Most look at the tread width equaling the rim width, so a 8.4 section width would probably have a 7.75 tread width and that will look a bit funny, not to mention being less than ideal on handling.

[Duke W.]

Re: Maybe a 225-70 or a 235-60 instead

Modern tires should be selected by matching revs per mile and load rating.

7.75-15, F70-15 and 215/70R-15 are all 775 revs/mile

6.70-15, GR70-15, 225/70R-15 are all 760 revs/mile

235/70R-15 is 744 revs per mile.

All these modern radials exceed the load rating of the OE tires. Models equipped with 775 rev/mile OE tires have the same speedo gearing as those with 760 rev/mi tires. GM just accepted the addtional +2% error, so installing a 760 rev/mi tire on a model originally equipped with a 775 rev/mile tire will reduce speedo error.

I recommend a 225/70R-15 police pursuit tires for all C3s, even '68s with 7" wheels as they are plenty wide for a 225/70.

For 8" wheels 235/70s should be considered, but there may be some clearance issues on some models. The higher the tire load capacity relative to vehicle weight, the better the car will handle and the more safety margin you have.

Duke

[Jim Callan]

Re: C3 tire size

Sorry - I guess that would help - '71 Coupe with OE wheels. I would like to use the 235/70 but concerned the tire won't fit in the spare tire carrier.

Thanks

Jim

[Duke W.]

Re: C3 tire size

Your original size was F70-15. Later C3s had GR70-15 and then 225/70R-15, and finally 255/60R-15. The GR70 and 225/70 have virtually the same dimensions. Don't know if the spare tire carrier was modified to accept these slightly taller tires.

The 235/70R-15 is a good "oversize" selection, but I don't know if it will fit the spare tire carrier.

Since it is rare for one to have to use the spare, you might consider using one of the existing tires as a spare. The issue you could face if the spare has to be mounted on the rear is torque pull and positraction clutch wear if the spare is not well matched to the other side tire's revs/mile. One way around this if you have a rear flat is to install the spare on the front and move the front to the rear.

In over 40 years of driving, I have never had to install a spare tire on the road. I've had my share of punctures, but they were slow leaks, and I was able to remove the tire and have it repaired at my convenience.

If you have the last couple of issues of The Corvette Restorer there is a two part article on all you ever wanted to know about replacement tires for Corvettes.

Duke

[Jim Callan]

Re: C3 tire size

Thanks Duke,

I'm replacing all 5 tires. The car had 255/60R-15's and they were OK, but now need to be replaced. The spare was a GR70-15 but is 25+ years old, even though the spare was never used, I don't feel comfortable using it for a spare. Also, when I removed it from the carrier, besides being flat, there was a depression
in the side wall. I'll probably go with the 225/70R-15's.

Thanks

Jim

[Joe L.]

Re: C3 tire size

Duke and Jim-----

The spare tire tub was the same for all 1968-82 Corvettes. However, I don't know if it will accomodate the 235-70-15. That tire has a greater OD than any tire ever originally used on a 68-82 Corvette so one would have to "experiment" to find out.

The difference in 68-82 tire widths was accomodated by different lock-bolt lengths. 3 different bolts were used over the 68-82 period.

[Todd H 26112]

Re: C3 tire size

What I don't understand is that the Judging Manual lists "215/70R15" as the "Correct current day service replacement size". The reason being the max rim width listed is 7.5" whereas we are talking about an 8" rim aren't why? Why does NCRS suggest the use of a tire size being mounted in a rim that may be 0.5" too wide?

It would sure be nice if a tire maker would offer a decent performance tire in 15" sizes in conjuction with some sort of good looking sidewall treatment that fits the look of a vintage performance car like the Corvette (etc). (No I do not wish to advertise for NASCAR or IRL)

Has the 'collect and show but rarely drive' thing gotten to the point that there is simply not a viable market to make it worthwhile for at least one major tire maker to offer a 'good looking' tire that is also a decent performer in 15" sizes? If so, perhaps we only have ourselves to blame. There is little beyond S and T rated tires in 255/60 for example which is a shame because that size puts a lot of rubber on the ground and looks good to boot. It's doubly disappointing because tires moreso than anything else save performance driving education can enhance a car's performance. Perhaps the continued popularity of SUVs/Trucks may yield some more options for us or sustain a few others. But I for one may be heading towards 16" rims.

[Duke W.]

Re: C3 tire size

215/70R-15 is the "correct" modern replacement size for F70-15s because they have the same revs per mile and other dimensions are close. The 225/70R-15 is the "correct" modern replacement size for GR70-15s and of course 225/70s became the standard size in 1978.

As far as modern replacements are concerned there are a few choices, and all you really need is one. The W-rated Pirelli P4000 Super Touring is a good choice in in a 215/70 and there are three V-rate police pursuit tires in 225/70 and 235/70. All of these tires have "all-season" compounds and tread design, which is somewhat of a compromise. My choice would be DOT legal racing tire type compound and tread design if they were available in suitable sizes.

The trouble is that most Corvette guys want "big" tires or white lettering. Tire width is not that big a factor in tire performance. It's all in the casing construction and tread compounds, but unless Corvette guys start demanding real tire performance instead of cosmetics, the vast majority of choices will be just low speed rated, white letter, low performance tires.

Duke

[Todd H 26112]

Re: C3 tire size

"15/70R-15 is the "correct" modern replacement size for F70-15s because they have the same revs per mile and other dimensions are close."

Well as stated above, the 215 matches RPMS (this actually varies modestly by manufacturer does it not?) but in "other dimension" the 225 is exact and correct where the 215 is not compared to F70. Rim width in your own article showed to be a dead-on match for the 225 @ '6-8 in.' just like the F70 and was notably shy on the 215 @ '5.5-7.5 in'. So are you saying a ~1% deviation in RPMs is MORE significant than a ~8% deviation in recommended rim width in selecting the correct service replacement?

Also I'm curious who designates what is the correct service replacement for defunct F70s? NCRS, GM or a major tire manufacturer or tire standards organization? Afterall GM itself end up w/ a 225/70 on an 8" rim on subsequent C3s.

"but unless Corvette guys start demanding real tire performance instead of cosmetics, the vast majority of choices will be just low speed rated, white letter, low performance tires."

Yes. And it's not just Corvette guys but most any vintage car population beit Mustangs, Camaros et al. But I'm not holding my breath at this point. I'll admit I'd like a tire that looks good AND performs well. That is, I want to have my cake and eat it too. While I appreciated your article - I simply feel highly constrained by 15" tire options compared to my other vehicles or desire to autoX again. And unless sustained by SUVs or law enforcement, even these options may dwindle and drop off. I'm likely heading the direction of a 16" rim somewhere down the line (off topic from this forum).

Maybe it's wishfull thinking but still I wonder if there might be a decent 'untapped' market out there for a good performance tire in 15" w/ an appropriate cosmetic treatment to it's sidewall. Say a police or police-like tire at a minimum w/ some sort of a cosmetic RWL treatment or stripe options or other for example.

"Tire width is not that big a factor in tire performance" - I've always understood with my limited experience w/ autoX and road events that the wider contact patch can be handy in lateral performance. In the past when I was more into this kind of thing I used to vary and stagger widths to get help get to the right mix of steering I was after. I for one have no qualms whatsoever about taking an original and mostly 'correct' vintage Corvette that is a flight judge candidate for a spin on the local AutoX course now and then.

[Duke W.]

Re: C3 tire size

Another factor in selecting "equivalent modern" tire sizes to replace out of production obsolete sizes is load capacity. The replacement size should have at least the load capacity of the obsolete OE size. The 215/70R-15 has a bit more load capacity than the F70-15 and the same OD and nominal revs per mile figure, so it is "equivalent" by any objective measure. My personal recommendation is to use at least on 225/70R-15 on any C3 with eight inch wheels, and I think NCRS should accept 225/70s on any C3, even those originally equipped with F70-15s, but it's something the '68 to '72 crowd is going to have to work if there is enough interest.

The wheel width range is based on an industry forumula for 0.70 aspect ratio tires. Exceeding it by 0.5 inch is not a safety issue, nor will a slightly underwidth wheel affect safety, but performance characteristics can be compromised with too narrow a wheel. Tall tires are less sensitive to wheel width than modern very low profile tires.

Over the last ten years I've talked to every tire manufacturer at SEMA about making sticky tires in sizes suitable for vintage car wheels. The response is usually a blank stare of maybe a snicker. Bottom line: It ain't gonna happen! The tire industry is capital intensive and margins are razor thin. If production numbers aren't in the hundreds of thousands or at least tens of thousands annually for specialty tires the major players are just not interested because there's no profit potential.

There IS a specialty tire for our Corvettes - the Michelin XWX that I discussed briefly in the tire article. I recall talking to only one person who was interested upon finding out about their existance and knowing they are over $1000 a set. Coker is selling so few 215/70VR-15 and 225/70VR-15s that they were talking about dropping those sizes when existing stocks are depleted.

Most muscle car and Mustang guys are more interested in doing burnouts than late braking before pitching into the corner, so for them, cheap hard compound tires are their cup of tea because they go up in smoke with little provocation.

Tire width is one factor in performance, but a no where near as important as most guys think. The biggest factor is tread compound and casing construction, and another is load capacity, which is roughly proportional to contained air volume. Given this, if you have two tires that are identical in all but section and tread width, the wider tire will have more load capacity due to its larger volume and for a given vertical load, will generated more grip.

Duke

[Terry M.]

Re: C3 tire size *TL*

Todd,
To chose the equivalent "modern" replacement tire I went to both Goodyear and Firestone and asked them for the equivalent to F70 15. Their answer, in the late 1980s when the 1970-72 manual was being revised, was 215 70R-15. At that time there were considerably more choices available in that size than there are today. Since then the performance tire market has gone to larger diameter and smaller aspect ratios, and the reproduction tire market has moved closer to OEM tires (although admittedly only one manufacturer has stepped up to the reproduction market).