Correct Equivalent for 775x15

[Stephen G.]

I keep getting different answers. If I'm putting radials on my 67 driver should they be 205/75/R15 or 215/70/R15?

[Duke W.]

Re: Correct Equivalent for 775x15

Either will work. It's a matter of reasonably matching load capacity, revs/mile, and overall diameter and section width. The 215/70 at 774 revs per mile is closer to the 7.75-15's 775 revs per mile. It's about the same inflated diameter (27.0"), but has greater section width. Plus you can buy a Z-rated 215/70 with the all important spiral wrapped nylon cap belt that gives you a HUGE safety margin against catastrophic tread separation. The minimum recommended wheel width is 5.5", so they are okay on OE C2 wheels.

The best speed rating you will find in a 205/75 is "S" and NONE in this size have the cap belt. The revs/mile figure is about 766.

205/75R-15s are made to the minimum DOT standards that have not changed in 40 years.

The choice is yours.

Duke

[Stephen G.]

Re: Correct Equivalent for 775x15

Thanks Duke.

Actually one of the guys in the club insists on 215/75/R15

[Duke W.]

Re: Correct Equivalent for 775x15

215/75R-15 is about 3/4" larger inflated OD and is speced at about 752 revs/mile. So is this closer to the 27.0" OD, 775 rev/mile 7.75-15 than the 205/75 or 215/70? Have the "one of the guys" do the math.

The 215/75 would be a good match for a car that originally was equipped with 7.10-15s if you are willing to accept a minimum quality tire.

Tire specs are on any tires mfgs. Web site, and they are also listed for all tires listed at tirerack.com. Just click "specs" for the specific tire you are viewing.

It ain't rocket science. The data is out there and easy to access.

Duke

[John H.]

Re: Correct Equivalent for 775x15

Thanks Duke.

Actually one of the guys in the club insists on 215/75/R15

I'd rely on the '67 TM&JG if you're looking for a judging-correct tire, not the "guy"; the JG specifically calls out 205/75-15 or 215/70-15 as the acceptable radial alternative sizes to the original 7.75-15 tires.

[Stephen G.]

Re: Correct Equivalent for 775x15

Thanks John. These are going to be my driver tires mounted on some repro bolt on's. When I judge the car I have some correct originals with the rallys that came on the car.

[Stephen G.]

Re: Correct Equivalent for 775x15

Duke

I was looking around on line and found several Rev values for the 775x15. Some were at 730, 731, & 736. You had mentioned 775 Revs for that size. Was it a typo or thats the info you have? The Revs for P70 & P75 all agree with what you had told me.

Steve

[Duke W.]

Re: Correct Equivalent for 775x15

The 775 revs/mile reference I have for the 7.75-15 is from some 40 year old tire brochures and handbooks. Also, I believe the AMA specs list the revs per mile for the OE tires.

Tire revs per mile are measured at design load and pressure and are specified by the manufacturer.

Many moronic Web site "calculators" compute the "rigid body" revs per mile based on unloaded inflated OD. This is valid for steel railroad wheels, but not for pneumatic tires because pneumatic tires deflect under vehicle load.

I have a four-inch thick file of tire data going back forty years, and I stand by it.

As a general rule of thumb, if you can't find an actual revs/mile spec, the tire loaded revs/mile is typically about 103 percent of the "rigid body" revs per mile calculation, and it will increase about two percent from full tread to the wear bars.

Duke

[Stephen G.]

Re: Correct Equivalent for 775x15

Thanks for your help

Steve