1962 Tires

[Joe Pennington]

Re: 1962 Tires

Terry, are you planning to run the ralley trim rings and center caps or the original full face hubcaps? I'm running the original hubcaps with original 5" wheels, with "nubs" on the front with 205/75's. Close to original diameter ( fills the wheel well) but easier to steer. On the rear, I'm running '67 "DC" ralleys which are 6" wide with the 225/70's. I had to weld the four "nubs" on the ralleys to hold the hubcaps. You can not use the original hubcaps on any wheel if you don't have the "nubs". Also, if you use wheels that were not manufactured in the 50's/60's, make sure the pilot hole is of the correct size. later wheels have a smaller hole, and will not fit your hubs. The 205/75's are 27.1" in diameter and the 225/70's are 27.4" in diameter. The original 6.70's were 27.4"/27.5" in diameter. Duke lists the tire revs per mile on several tire sizes but looking at old Road & Track road tests they varied quite a bit. R & T, August 1957 issue, 1957 Corvette with 6.70 tires with 4.11 rear gear shown as engine revs/mile of 2960 which equals 720 revs/mile of the tires. R & T, October 1962 issue, 1963 Corvette with 6.70 tires with 3.70 rear gear shown as engine revs/mile of 2750 which equals 743 revs/mile of the tires. R & T, August 1965 issue, 1965 Corvette with 7.75 tires with 3.70 rear gear shown as engine revs/mile of 2820 which equals 762 revs/mile of the tires. R & T, January 1968 issue, 1968 Corvette with F70 tires with 2780 engine revs/mile which equals 751 revs/mile of the tires. 760 revs/mile equals 13.3" loaded wheel radius and 775 revs/mile equals 13.0" loaded wheel raduis. I measured the loaded wheel radius my cars, at 35psi, and found the following. 205/75-15 = 12.5" = 806 revs/mile. 205/70 = 12.3 = 817 revs/mile. 225/70-15 = 12.9" = 782 revs per mile. I have the 205/70's(new) on my '64 with 6" wide K/O's and when checked against my C5 with less then 5,000 miles on the tires, the two cars were within 1/2 mph of each other at 60mph. Either my C5's speedo is way off, which I doubt, or the tire companies specs are off.

[Duke Williams (22045)]

Re: 1962 Tires

Revs per mile will vary with load and pressure. I believe published specs are based on measurements at maximum load and pressure. The testing is done on laboratory test beds, and I'm not sure that all sizes are tested individually. Some sizes revs per mile may be based on the "curve", actually a straight line, that is formed when revs per mile is plotted against static loaded radius. Also, I have seen manufacturers state that specs can vary up to two percent due to production tolerance.

Also, specs are for new tires with full tread. For a typical tire worn to the tread bars, the revs per mile will increase about two percent.

Tires aren't like cranshafts when it comes to their dimensions and other specifications!

Regarding the data from the old car mags, I'm not sure how that data was determined, but I suspect it was from specified or measured tire diameter used to calculate "rigid body" rolling circumference, so it's not that accurate. It could also have been derived from measured data using their old mechanical fifth wheel test speedomenter and a test tach, but that equipment was probably only accurate to within two percent at best.

Now days they use a radar based system that measures the rate that the ground is passing beneath the car, which is probably accurate to well under one percent.

Duke

[Joe Pennington]

Re: 1962 Tires

Duke, I believe most of the tests back then used the fifth wheel to measure speeds, speedo accuracy, acceleration, etc. What I found strange was the amount of varance of the tire revs/mile being that the tests were probabily done with the same equipment since it was all from the same magazine. As we all are aware, radials are more sensitive to load and air pressure then non-radial tires when it comes to the LWR. I didn't check the FWD Buick, but it wouldn't surprise me if the LWR on the front tires was as much as 3/8" less then the rears. I would guess that the LWR of a 6.70-15 tire would be about 3% less then the non loaded radius and a radial LWR would be around 6% less, on the same car, same axle. I can't explain why a 205/70-15 radial at 817 revs/mile on my '64 would give me the same speedo reading as a 6.70-15 tire spec'd at 760 revs/mile. That's a 7.5% differance or 4.5 MPH at 60MPH vs. the 1/2 MPH I measured against my C5. I don't have a 6.70-15 mounted on a car but maybe some one who does can measure the LWR. At 760 revs/mile it should be 13 & 17/64's.