Tires again

[Ed P.]

P 205/75 R15 Is this the correct tire size for the front tire on my 62 ?
I read in the 62 restoration guide that 6.70x15 is the original size for the 62.

Being a dumbie are these the same tire size ?

edd

[John F.]

Re: Tires again

I run these on my 62's.

[Leif A.]

Re: Tires again

P 205/75 R15 Is this the correct tire size for the front tire on my 62 ?
I read in the 62 restoration guide that 6.70x15 is the original size for the 62.

Being a dumbie are these the same tire size ?

edd

Ed,

Try this http://www.vintagecarconnection.com/...sion_chart.htm

[Joe L.]

Re: Tires again

P 205/75 R15 Is this the correct tire size for the front tire on my 62 ?
I read in the 62 restoration guide that 6.70x15 is the original size for the 62.

Being a dumbie are these the same tire size ?

edd

Ed------


They're not the same size nor do they have the same dimensional specifications. However, they are "close" and fitment has been proven.

In my opinion, the 215-75-15 is a closer size to the 6.70-15. However, I have no tire fitment experience with C1 Corvettes and I don't know if some fitment problem has been observed with the 215-75-15.

Keep in mind that with either the 195-75-15 or the 215-75-15, tire choices are going to be very limited and NO really high performance tires are going to be available. 75 series tires are a "dying breed".

[Duke W.]

Re: Tires again

The important thing is to match the original tire revs/mile (to maintain speedometer accuracy), which can only be determined by testing, not by calculating the static rolling circumference using the inflated OD, which is what most Web based calculators do, SO THEY ARE ALL WRONG!!!

The OE tire size revs/mile is listed in the AMA specs. Tire manufacturers specifiy the data, and it's posted on tire retail Web sites and tire manufacturers' Web sites. The allowable revs/mile for each size are in a very narrow range as specified by the Tire and Rim Association, so you can pick any brand and be within a fraction of a percent of any other.

All OE 15" tires are (nominally) either 760 or 775 revs/mile, and the 205/65, 215/70, and 225/70 are all within this range, so they are the three choices for current radials, but all 205/65s are cheaply constructed, low speed rated tires that lack the safety of a spiral wound nylon cap belt.

Simple, right?

Duke

[Joe L.]

Re: Tires again

The important thing is to match the original tire revs/mile (to maintain speedometer accuracy), which can only be determined by testing, not by calculating the static rolling circumference using the inflated OD, which is what most Web based calculators do, SO THEY ARE ALL WRONG!!!

The OE tire size revs/mile is listed in the AMA specs. Tire manufacturers specifiy the data, and it's posted on tire retail Web sites and tire manufacturers' Web sites. The allowable revs/mile for each size are in a very narrow range as specified by the Tire and Rim Association, so you can pick any brand and be within a fraction of a percent of any other.

All OE 15" tires are (nominally) either 760 or 775 revs/mile, and the 205/65, 215/70, and 225/70 are all within this range, so they are the three choices for current radials, but all 205/65s are cheaply constructed, low speed rated tires that lack the safety of a spiral wound nylon cap belt.

Simple, right?

Duke

Duke------

The 6.70-15 tire is about 710 revs/mile.

[Terry D.]

Re: Tires again

Here are two web sites that you can use to determine tire size. http://www.miata.net/garage/tirecalc...m/specspro.jsp

Terry

[Terry D.]

Re: Tires again

http://www.rimsntires.com/specspro.jsp

This is the second one, don't know why it didn't go thru correctly. The first one ends with the tirecalc.

[Duke W.]

Re: Tires again

Duke------

The 6.70-15 tire is about 710 revs/mile.

The AMA specs from the 6.70-15 era list revs/mile at 760. Where did you get the 710 number?

Here are the typical loaded revs/mile for all OE 15" tires and current replacement radial sizes as listed in AMA specs and tire manufacturers' specs:

760: 6.70-15, GR70-15, 225/70R-15

775: 7.75-15, FR70-15, 215/70R-15

766: 205/65R-15

From new to the wear bars revs/mile will increase about two percent.

The calculated revs/mile from the Miata Web site calculator is wrong for the reason I stated in my earlier post. It's based on the "rigid body" calculation using the inflated OD. Actual revs/mile of a loaded pneumatic tire is greater due to deflection, typically by about three percent.

Rather than rely on Web calculators, all of which I've seen are wrong, just go to The Tire Rack Web site, look up the size, pick any make/model, click on "specs" and get an accurate value.

We've had this discussion multiple times.

Duke

[Dennis O.]

Re: Tires again

Duke,

You said : " It's based on the "rigid body" calculation using the inflated OD. Actual revs/mile of a loaded pneumatic tire is greater due to deflection, typically by about three percent." I am confused by this. As I see it, once inflated, the circumference of the tire assumes a constant value, determined by the structural fabric and the tread material. Given no slippage between the road and the tire tread, how can deflection cause this distance to vary?

[Joe L.]

Re: Tires again

. Where did you get the 710 number?




Duke

Duke------

Kelsey Tire specifies 713 revs per mile for their reproduction 6.70-15 tire

[Duke W.]

Re: Tires again

Some manufacturers specify "static loaded radius", which is less than the unloaded radius that is one half the specified inflated (unloaded) OD. You can clearly see the deflection of a loaded tire, particularly the bulge in the sidewall of loaded radials at relatively low pressure, which reduces the static loaded radius from the unloaded radius. This static loaded radius is effectively the rolling radius, and since it's less than the "rigid body radius", revs/mile are higher.

A steel railroad wheel's rev/mile can be calculated from the unloaded OD because deflection is noise level, but this is not the case with pneumatic tires because deflection under load is significant.

For example the 215/70R-15 size typically has a specified inflated OE (no load) of 27.0" (13.5" radius), which yields an unloaded circumference of pi x 27 = 84.82" or 7.069', so it will take 747 revolutions, assuming it's a rigid body like a steel railroad wheel, to roll 5280', which is one mile.

The typical loaded revs/mile spec is 775 - 3.7 percent greater - and corresponds to a static loaded radius (or rolling radius) of 13.0".

Loaded rolling radius is usually specified at the maximum placarded load and maximum placarded cold inflation pressure, so actual loaded radius on the car will vary slightly from this depending on load and inflation pressure, but to obtain accurate speed vs. revs data, the specified revs/mile will yield very close results, and it's the baseline to use for checking speedometer and odometer accuracy.

Duke

[Duke W.]

Re: Tires again

Duke------

Kelsey Tire specifies 713 revs per mile for their reproduction 6.70-15 tire

Current reproduction bias ply tires are larger in diameter than back in the fifties and sixties, which is why many won't fit in the spare tire wells of C1 and C2 Corvettes. I believe John Hinckley posted some time ago that DOT regulations require the tire to be inflated hot out of the mold, which increases static OD (and loaded rolling radius) relative to period manufactured 6.70-15s. Also, how can we be certain that Kelsey is listing revs/mile based on actual loaded testing or just quoting the number based on rigid body calculations?

If they also list static OD you can do the calculation to determine. My bet is they are just using the rigid body calculation rather than actually testing loaded revs/mile. It has to be actually tested, or at least estimated by adding three percent to the rigid body calculation.

In order to maintain reasonable speedometer and odometer accuracy it's important to match the loaded revs/mile spec of originally installed tire sizes, not modern reproductions.

As I stated previously the AMA specs from the 6.70-15 era specify (loaded) revs/mile at 760.

Duke

[Dennis O.]

Re: Tires again

Duke,

I am still unclear on this. First of all, I don't think radius has anything to do with it, it is the circumference that is the determining factor. Say you have a tire with a circumference of 90 inches.. In one revolution it will travel 90 inches. Now say you have a small caterpillar tread, also 90 inches in circumference. It ill also travel 90 inches with one full revolution. The distance from the center of the tire to the ground (it's radius) will be a lot more than the distance between the centerline of the caterpillar track and the ground, yet they will both travel 90 inches with one full revolution. I can't believe there is 3% slippage with every revolution of a slightly deflected tire. If this were the case, we would be buying new tires a lot more often when we do.

[Duke W.]

Re: Tires again

The increase in revs per mile for a loaded pneumatic tire compared to rigid body like a steel railroad wheel is due to deflection, not "slippage". The rolling circumference of a loaded tire is computed from the static loaded radius (which is less than the unloaded radiaus), not the unloaded radius, which is half the specified (unloaded), inflated OD.

A caterpillar tread is essentially a rigid body, like a steel railroad wheel, so if the tread is 20 feet long, one complete circuit will move the vehicle 20 feet. Likewise one revolution of a 42" steel railroad wheel will move the vehicle pi x 42" = 131.93" along the track.

Pnuematic tires are not rigid bodies. They have significant deflection, when loaded, which decreases the loaded radius/rolling circumference.

Duke