wheels & tires

[Larry Demry]

1965 Corvette; 327/350; close ratio 4-speed; 4:11 rear end; original

I would like to run a few less revs while highway driving. I can accomplish this by using bigger wheels and taller tires. What is the recommended wheel & tire combination to create the least revs? What is the formula where I can input one number (gear ratio, RPM, tire diameter, MPH, etc.) and calculate any of the other numbers? Thanks for your help.

[John H.]

Re: wheels & tires

Larry -

I doubt if you have clearance to run appreciably larger overall diameter tires - at least large enough to make much of a difference in overall effective final drive ratio. This is a difficult compromise - you can't go as low (numerically) as you'd like to with the close-ratio gears, although I run a 3.55 axle with an M-21 in my '69 Z/28 (it was originally a 4.10 and I changed it for more civilized highway cruising too). Anything lower (numerically) than a 3.55 and I'd have to change to a wide-ratio Muncie, or buy stock in a clutch manufacturer. Practically speaking, you're looking at a rear axle ratio change, and possibly a trans ratio change as well. We're spoiled with the overdrive 5- and 6-speeds on modern cars, but there's no free lunch with a 4-speed with a 1:1 top gear.

[John H.]

Re: wheels & tires

Larry -

I doubt if you have clearance to run appreciably larger overall diameter tires - at least large enough to make much of a difference in overall effective final drive ratio. This is a difficult compromise - you can't go as low (numerically) as you'd like to with the close-ratio gears, although I run a 3.55 axle with an M-21 in my '69 Z/28 (it was originally a 4.10 and I changed it for more civilized highway cruising too). Anything lower (numerically) than a 3.55 and I'd have to change to a wide-ratio Muncie, or buy stock in a clutch manufacturer. Practically speaking, you're looking at a rear axle ratio change, and possibly a trans ratio change as well. We're spoiled with the overdrive 5- and 6-speeds on modern cars, but there's no free lunch with a 4-speed with a 1:1 top gear.

[Duke W.]

Re: wheels & tires

To add to what John said, the original tires are speced at 775 revs per mile and are about 27" in diameter. To equal the effectiveness of a change to a 3.70 axle, which is ten percent you would have to go to a tire with just under 700 revs per mile, which would be about 30 inches in diameter.

Off hand I can even think of a 15" tire that is that tall, other than some mud tire for off-roaders.

From a practical standpoint you can only get about a five percent change in effective gearing by juggling the installed tire revs per mile. Beyond that you will not have enough wheelwell clearance or your car will look like a low rider.

Duke

[Duke W.]

Re: wheels & tires

To add to what John said, the original tires are speced at 775 revs per mile and are about 27" in diameter. To equal the effectiveness of a change to a 3.70 axle, which is ten percent you would have to go to a tire with just under 700 revs per mile, which would be about 30 inches in diameter.

Off hand I can even think of a 15" tire that is that tall, other than some mud tire for off-roaders.

From a practical standpoint you can only get about a five percent change in effective gearing by juggling the installed tire revs per mile. Beyond that you will not have enough wheelwell clearance or your car will look like a low rider.

Duke