any know what are todays sizes for F70-15 & GR70-15 tires. Iv'e called 10 local tire stores and have gotten 10 different answers. obiously 15 is 15 inch but iv'e had the other translated everywhere from P205-70R15 to P235-70r15.
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Re: what are todays sizes for F70-15 & GR70-15 tir
geno------
There is NO modern tire size which is exactly the same as either the F70-15 or GR70-15 in all dimensions. For the F70-15, I consider that the P215-70R15 is the closest in OVERALL dimensions. It's about 1/4" narrower in section width, but very close to the same in OD. The problem is that the 215-70R15 is not usually recommended for wheel widths larger than 7", but, in my opinion, it can be safely used with 8" wheels. The P225-70R15 is the other choice. It's about the same section width as the F70-15 but it's about 1/2" larger in OD. So, using the 215-70R15 will give the best shot at maintaining accurate speedometer readings.
For the GR70-15, I think that the P225-70R15 is clearly the best choice. The P235-70R15 is too large.
The real problem is that getting any of the above sizes in a real hi-performance tire is getting to be well-nigh impossible these days. But, you can still certainly get tires that have a far greater performance potential than the original tires that 60s and 70s Corvettes were fitted with.In Appreciation of John Hinckley -
Re: what are todays sizes for F70-15 & GR70-15 tir
joe what would us poor humble folk do without you. thanks.Comment
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Re: what are todays sizes for F70-15 & GR70-15 tir
To two important parameters to match are REVS PER MILE, and LOAD CAPACITY.
Based on these two criteria the 215/70R-15 (775 revs/mile) is the correct modern substitute for the F70-15, which has the same revs/mile spec, and the 225/70R-15 (760 revs/mile) is essentially an exact match for the GR70-15; 760 is also the revs per mile spec for 6.70-15s.In both cases the modern radial's load capacity exceeds the vintage original.
Tires in these sizes are available in "all-season" high performance versions with speed ratings up to "Z". Unfortunately, "summer" high performance tires, which have more grip in warm temperatures are not available in these sizes, but Firestone has a 225/70 that they market as a "police pursuit tire".
For all Sharks with seven or eight inch wheels I recommend the 225/70R-15.
For C2s I recommend the 215/70R-15, and it's best to mount them on KOs or six inch Rallys as the 5.5" steel wheels are a bit narrow. As far as I know the 215/70s will fit on any C2 with any of the OEM wheel widths and offsets with sufficient clearance to precude fender damage, but they might rub the heads of the steering box carriage bolts at full left lock.
Running a bit of negative camber on both ends (minus half a degree up front and minus a half to one degree in back) will increase fender clearance.
DukeComment
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Re: what are todays sizes for F70-15 & GR70-15 tir
i dont know where you guys come up with this information but thank god for the ncrs and giving me access to you folks. i appreciate it.Comment
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It's not rocket science
Matching the OEM revs per mile will keep your odometer and speedometer within reasonable accuracy limits, and load capacity is just plain common sense.
Nearly all the tire manufacturers web sites have charts that list "engineering" data for all their tires including revs/mile and load capacity.
About eigthty percent of the handling equation is tire grip and adhesion.
I figured this out while I was still in high school. A few months after taking delivery of my SWC I took it to Kent for some hot laps and chunked the OEM General Jetaires. This lead me on a search for the best available tires, and at the time, circa '63 to '64, in came down to the Goodyear Blue Streak Sports Car Special (a purpose built racing tire), The Pirelli Cinturado radial, and the Michelin X radial.
I went with the Michelin because it appeared to have the best combination of wet and dry grip and longevity. A lot of people laughed at me for buying those "funny French tires", but the last laugh was on me as there was NOTHING short of another Michelin shod Corvette that could keep up with my SWC on either dry or wet roads, and being as how I lived in Seattle at the time, wet performance was an important criterion. I even passed stuck Corvairs and Volkwagons going up some of Seattle's hills when it snowed. Those tires were utterly amazing compared to the bias ply tires of the era.
I've been paying close attention to tires and tire technology ever since.
DukeComment
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Re: what are todays sizes for F70-15 & GR70-15 tir
Example...225 70R15 225 is the tread width in Millimeters.
70 is the aspect ratio or wall height (70% of the tread width).
R is "radial construction".
15 is rim diameter.
So take 225 + 2 X (70% of 225) , multiply by 25.4 to get inches and the add rim diamete (15). This will give you the Diameter of the tire and from that you can pick a tire that will be as clos as possible. Hope this helped.Comment
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Question Duke...
Duke,
were those Michelins the same as the XWX's available today for vintage cars? Thanks, George #31887Comment
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Re: Question Duke...
No. The Michelins I installed on my SWC in '64 predated the XWX by nearly a decade. The tires I installed were called simply "Michelin X" radial, size 6.70-15, and were not speed rated, but they survived at least a half dozen hot lap sessions and a couple of runs to 150 MPH. They lasted 47,000 miles! Coker offers this size, but with a more "European" like tread pattern, with three central zig-zag grooves. The tires I installed had a more conventional "American" tread pattern that was similar to contemporateous bias ply tires of the era. They are about 27" in diameter and have a tread width of about 4.5 inches. I still have one that I use as the spare.
The XWX available from Coker is the exact same legendary tire from the early seventies. At SEMA last year, Coker reps assured me that it is built with the original molds and materials. Michelin won't do it any other way, they said. The XWX in the 215/70VR-15 and 225/70VR-15 are likely THE BEST performance tires available for vintage Corvettes today, but unfortunately, these sizes are not selling well, and Coker may not reorder when existing stocks are exhausted. The were OEM on Ferraris (like the Daytona) and V-12 Lambos.
The 185/70VR-15 XWX is selling well to Porsche restorers, and will likely be around for awhile. The Corvette sizes cost about a thousand bucks a set and will likely only last 20-25K miles, but I don't think there's a better performing tires out there in the 215/70 and 225/70 sizes.
DukeComment
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Re: what are todays sizes for F70-15 & GR70-15 tir
Larry,
To get tire diameter (in inches), you simply add 15 inches (if that's the wheel diameter) to twice the sidewall height (in inches). On the example you used, with a sidewall height of 70% (aspect ratio) of 225 millimetre tread width, you would add twice the product of: .7 x 225 DIVIDED by 25.4 (to convert to inches) to the wheel diameter of 15 inches.Comment
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Re: what are todays sizes for F70-15 & GR70-15 tir
Your calculation will provide a nominal OD, but computing the revs per mile from the inflated OD is not a good way to select a tire. The actual revs per mile are greater because of distortion.
It's okay to match railroad wheels by diameter. They are "rigid bodies" for all intents and purposes, but pnuematic tires are not.
The revs/mile data are published by virtually all tire manufacturers and are listed on their web sites, so there's no real reason to use anything other than specified revs per mile if you want to select a tire that is close in true rolling radius to the OEM tire.
The Tire and Rim Association promulgates dimension standards for each nominal tire size and the range is quite narrow - about two or three percent, so once you get the specs for a specific make and model size, you'll generally find that all other manufacturers same sized tires will be speced within about one percent.
As a quick reference the following are the typcial revs per mile for Corvette OEM tires and recommended replacement sizes.
6.70-15.......760
7.75-15.......775
F70-15........775
GR70-15.......760
205/65R-15....768
215/70R-15....775
225/70R-15....760
It's my understanding that manufacturers spec revs per mile at max load and cold inflation pressure, so different loads and pressures will affect the actual figure on your car by a very small amount. Also, the revs per mile will increase with tire wear - on the order of two percent when the tread is worn to the wear bars.
Duke
DukeComment
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Re: Question Duke...
The XWX's are expensive, but they are terrific tires - incredible grip, with the expected trade-off in wear resistance. They were OEM on the Ferrari 275GTB/4, 330GTC, and 365GTS/4's I had in the early 70's, in 205VR14 and 215VR14 size (equivalent to the currently offered 205/70 and 215/70). They are currently available in 15" diameter in 195, 205, 215, and 225 sections.Comment
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Re: what are todays sizes for F70-15 & GR70-15 tir
OOPS! I did mean divide by 25.4 Brain fade.... What can I say>Comment
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Re: what are todays sizes for F70-15 & GR70-15 tir
Duke,
Out of curiosity, what would the revolultions be for size: P235 60 15?
Would that size affect speedo reedings?Comment
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Re: what are todays sizes for F70-15 & GR70-15 tir
I don't know the revs/mile spec for that size off the top of my head, but you can check any manufacturer's web site that offers that size. Then compute the percent difference between that an the OEM tire. That's the additional speedo/odo error you will pick up.
DukeComment
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