Need gear calculator, Rpm vs mph?
05-23-2009, 12:57 AM
#1
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Need gear calculator, Rpm vs mph?
I have 3.73's and a tr3650. I would like to know what my mph would be at my 7200 rev limiter in 4th? Wondering if I have enough gear for the 1/4 without going to 5th. Also I shift close to 7k and was wondering if I should raise the limiter a little because I bump it sometimes?
05-23-2009, 01:05 AM
#2
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Found this one but I not sure how accurate it is. It says my 315/35/17's have an OD of 24.91" and I thought they were closer to 26". Plus it only goes to 7k rpm.
http://www.paladinmicro.com/PalMFrame00.htm?wks=spd
http://www.paladinmicro.com/PalMFrame00.htm?wks=spd
05-23-2009, 05:34 AM
#6
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its pretty accurate if you just don't read the table all the way to the end, the MPH isn't that the reason why they tell you to use taller tires so you don't run out of gear at the track?
For the OD diameter for that specific tire I get 25.68in for the 315/35/17 so its only off by 3%. I'm sure there is a reason for it because Cliffy seems like a real smart guy, knows his stuff.
For the OD diameter for that specific tire I get 25.68in for the 315/35/17 so its only off by 3%. I'm sure there is a reason for it because Cliffy seems like a real smart guy, knows his stuff.
05-23-2009, 06:05 AM
#7
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325/30-17, with 3.73s and a 3650, calculates to 184.8 in 5th @ 6k RPM;
325/50-17, with 3.73s and a 3650, calculates to 223.1 in 5th @ 6k RPM;
Re: tire size;
315/35-17s have a 110.25 mm high side wall (315 mm * 0.35). This times 2 = 220.5 mm = 8.68" + 17 " = 25.68" -- however only 97% of that is used in the calcuations because loaded tires are "flat" on the bottom, which reduces the effective radius--this is explained in the footnote at the bottom of the page:
1 - 97.0% of actual diameter is used in the RPM calculations to more accurately reflect the real world performance of a loaded tire. This is of course only an imprecise guesstimate of the actual effective diameter--which is influenced by vehicle weight, tire inflation pressure, and other factors--nonetheless I have found it to be reasonably accurate.
Next time you're on the highway going 70 (or 150) look at the tires on the car next to you, they will be "flattened out" at the bottom, reducing the drive radius. If you use the theoretical calculated diameter in speed calculations, or to determine the revolutions per mile to set up a tune, the real life performance will be different.
Last edited by cliffyk; 05-23-2009 at 06:32 AM.
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