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View Full Version : Gears, tires and engine speed



rumble
11-30-2004, 07:47 PM
I keep noticing that questions re. RPM's and gear ratios and
tire sizes keep coming up over and over. The best link I have
seen to calculate the effect on engine speed is http://www.averagegeek.com/Evil_Jim/GearCalc.asp
This was once listed here on the site but seems to have disappeared.
In fact as many times as these questions come up I think
this link should be considered a "sticky"

TripleTransAm
11-30-2004, 08:36 PM
And if you don't have access to a computer, you can use a simple calculator, if all you're interested in is the change in RPM when changing one item.

It's a matter of ratios between the old size / ratio and the new one.

If changing gear ratios, note the RPM at any given speed you're interested in, and multiply it by the ratio of the 2 gears. If you have 3.55 gears and are doing 2000 RPM and want to know what RPM you'd see with 4.10s, the answer is:
2000 RPM x (4.10 / 3.55) = 2310 RPM

Works with tranny gear ratios too... let's say you're doing 3000 RPM cruising in 3rd (ratio = 1:1) and you enable OD (4th gear = 0.70:1, I think). The new RPM is:
3000 RPM x (0.70 / 1.00 ) = 2100 RPM
(of course, this is assuming the TCC is locked up, or there will be a slippage introduced and the numbers won't be exactly related by gear ratios alone).

Downshifts are fun, too... cruising in 4th at 2100 RPM and slam the shifter into 2nd gear (1.55:1)?
2100 RPM x (1.55 / 0.70 ) = 4650 RPM
(although again it may be higher if the TCC unlocks)

Same with changing the diameter of the tire... find the ratio between the diameter of the new tire and the old one, and apply it to your speed at a given RPM to find the new speed.

studio460
12-02-2004, 05:16 AM
Hmmm . . . I made a spreadsheet for all of this once (which I was planning to post here somehow), but the HDD it was on failed.