295/55R15

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K.P.
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295/55R15

Post by K.P. » Fri Feb 12, 2016 10:33 am

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

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Re: 295/55R15

Post by Kenova » Fri Feb 12, 2016 5:36 pm

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

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Re: 295/55R15

Post by K.P. » Fri Feb 12, 2016 7:11 pm

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?

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Jer73
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Re: 295/55R15

Post by Jer73 » Mon Feb 15, 2016 1:04 am

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/

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Re: 295/55R15

Post by F-BIRD'88 » Sat Feb 20, 2016 3:22 pm

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"

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Re: 295/55R15

Post by F-BIRD'88 » Sat Feb 20, 2016 3:25 pm

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?

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Re: 295/55R15

Post by K.P. » Sat Feb 20, 2016 9:56 pm

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.

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Re: 295/55R15

Post by K.P. » Sat Feb 20, 2016 10:02 pm

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

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