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Old 03-03-2009, 09:05 PM
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Question about tire size and rpm

So here is the scenario. Car is a 1980 camaro z28 with a TH350 3-speed a 383 stroker (that I built for it) and the rear end is stock with 3.52 gears. The rear rims are stock as well at only 14" wide and the tires mounted on them are fairly low profile with small sidewalls. Now with all this in mind just imagine for a second how bad this thing is on the highway!! Since it has fairly high gears in the back (3.52) and no overdrive in the tranny with very small tires on the rear it maxes out at like 80-90mph!! My objective/question with this post is to get this car a little more highway friendly so I'm not cruising at 4,500rpm in 3rd as I go down the highway!! I want to keep the tranny in there (a 700R4 would be nice but I ain't pulling the TH350 out again) as well as the rear end. I think the major problem here is my rear tires are ridiculously small (in diamter) considering they are on 14" rims!! Now for my question. Say my tires on the rear are 20" in diameter and at 90mph I'm at 5,000rpm (these aren't the actual #'s just an example), if I were to have 40" diameter tires on the rear would this effectively cut my rpm in half given everything else is the same?? When I logically think about it it makes sense to me, a 40" tire would travel twice the distance then a 20" tire in one revolution meaning the rpm would change in direct relation to tire diameter given the rear end ratio is still the same. I am looking at getting a nice new set of rims and tires for the car, stepping up to a 17" rim with some tires with larger sidewalls for the rear would durastically increase my tire diameter. All I want to know is if my theory on this is right, it may only lower the rpm by a thousand tops but that would be better than nothing. Please let me know if all this info is correct and what you think.

Thanks,
Keith

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Old 03-03-2009, 09:15 PM
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this should help you
http://www.dakota-truck.net/TIRECALC/tirecalc.html
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Old 03-03-2009, 10:09 PM
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Your car can only go 80 mph with 3.50 gears?

You have 14" wide tires or 14" rims?

I just did the calculation for 245/50-14 tires at 6000 rpm with a 3.5:1 gear ratio and the speed is 120 mph. That tires size is likely the largest tire you can fit without interference issues in a stock wheelwell.

Working backwards in the equation if you can only do 80 mph your rpm should be 4000 rpm, 90 mph would be 4500 rpm. This is with the transmission in top gear 1:1.

A healthy 383 should be able to top 5000 rpm in top gear quite easily with such a light car. Pretty sure the stock 6 that came in those cars did 100 mph, I was there when they first came out.

Sumptin ain't soundin right bud.
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