Don Terrill’s Speed-Talk

Hello all,
Would somebody tell me how much of a change in tire diameter there would be from changing from a 295/65R15 M/T Drag Radial to a 295/55R15
Thanks

The 55 series tire will be approximately two inches smaller in diameter.

Ken

Thank you for the reply Ken,
The reason I was thinking of going to a 55 series is because now I am running a 4.30 gear and going thru the traps (1/8th mile) at about 7000 rpm on a 489 BBC. Not really enough time to get it into 3rd gear (turbo 400). How much would 2 inches in diameter make gear ratio wise. Enough to see or feel a difference?

This may give you a rough idea of going from a 65 to a 55 series sidewall. Just input your trans ratios, rear gear, shift points, and tire size.

http://www.blocklayer.com/rpm-geareng.aspx

A 65 series is 30.1inch in diameter and a 55 series is 27.8inches according to this calculator.

https://tiresize.com/calculator/

30" tire to a 27.7" tire.

7000 to 7600 rpm same MPH

Thats the same as changing the 4.30 gears to a 4.65:1 gear ratio.

Math 295 / 25.4 x .65 x 2 /15 = 30"

295 / 25.4 x .55 x 2 / 15 = 27.7"

Are you crossing the line in the 1/8th in 2nd gear at 7000rpm now? What trap speed MPH?

What gear and what rpm in that gear do you want to trap at?

7000 rpm is pushing the envelope for me, I can short shift it into 3rd but it all really happens in the traps which to me is useless. I was thinking that is I used a shorter tire that I could get into 3rd a little earlier, but would it make any difference. 4.30 to 4.65 is not much of a change.
6.93 @ 100.35 mph/7100 rpm weight is approx. 3600 lbs this is with a slick 11 inches wide and 30 inches in diameter.
I hated the way the car felt with the slicks on it, it was scary to me as if the car was skating the whole 1/8 mile and I was going to lose it!
Maybe a 5.13 gear would be better? That wouldn't be any better if I couldn't get it to hook! I run a chrome 15X10 rally rim in the back with the drag radials, and about 15 psi. The rim would spin inside the tire and make a weird thumping noise when I left the line. I really didn't want to use rim screws and ruin the looks of the rally wheels so maybe some different wheels. Somebody once suggested that I just screw the inside bead but that didn't sound like a good idea either.

Yes, 2nd gear (turbo 400) @ 7000 rpm. I would like to trap 6700 rpm in 3rd gear.