Determining Engine RPM Formula??

[Dick G.]

What is the forumula for determining what RPM my engine will be running at 70MPH if I know my transmission and rear end gear ratio? If my 4L70E auto transmission 4th gear ratio is .070, and my rear end gear ratio is either 2.59, 3.08 or 3.54 what would the RPM of my engine be? Trying to determine what rear end ratio I want in this corvette.
Thanks DG

[Donald L.]

Re: Determining Engine RPM Formula??

http://www.angelfire.com/fl/procrastination/rear.html

[William C.]

Re: Determining Engine RPM Formula??

One other item you need is the loaded radius of the tire from axle center to the ground

[Duke W.]

Re: Determining Engine RPM Formula??

Revs @ 60 = tire revs per mile x axle ratio x trans ratio, and use linear interpolation from there.

Tire revs per mile for most tires is listed in manufacturers' spec sheets, which are available on their web sites or retailers like The Tire Rack. If you can't find the specific model size, use any same size specs as revs per mile for each size is in a very narrow range by industry standard, and the revs per mile for typical OE and replacement sizes have been listed here many times.

Duke

[Jean C.]

Re: Determining Engine RPM Formula??

Back in the day, you could buy a small circular slide rule that calculated either speed, rpm, gear set or tire size on one side and maybe something else on the other side. One of the hot rod parts vendors marketed them as I vaguely recall...but that was a long time ago. Isky? Edelbrock? Good ol' Honest Charley?
Cheers,

[Dick G.]

Re: Determining Engine RPM Formula??

Typo: the 4th gear ratio of the 4L70E is .70 not .07. I will be using my LS2 crate engine mated up to my 4L70E. I was advised today not to buy a Richmond ring and pinion. I know someone who has several rebuilt rear ends that have a loud whine all with the Richmond gears. Anyone else? Dave: where do you get your gears? With the 4th gear of the tranny at .7 18inch tires measuring at 25.6 height at 70 mph: rear end ratio of 2.59=1666rpm, 3.07=1974, 3.54=2277, 3.73=2399rpm of engine. Look Right? Which ratio is desirable. I will not be racing this car, street only. I was told that the rear end was a 3.07. It is a 2.59. That's another story. Anyone have an 88-96 Dana 36 with at least a 3.07 ring and pinion? Thanks DG