MPH = RPM

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Oct 21, 2016 | 07:45 AM

Jim and Austin

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Joined: Dec 2015

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From: SE Ohio 45629

MPH = RPM

Since starting our revitalization our grandsons truck I have been concerned about the 4:27 rear axle ratio. I ran across this formula the other day that would mathematically state the engine RPM at a specified speed. I thought it was interesting and worth sharing. Hope it helps someone else on the site!

Here is the formula:

Speed X gear ratio X 336 / tire diameter

So Austin's truck would calculate as follows:
50MPH X 4.27 X 336= 71,736 / 28.5 = 2517 rpm

Same thing with a 3.91 ratio:
50 X 3.91 X 336 = 65,688 / 28.5" = 2304 rpm

Same thing with a 3.73 ratio:
50 X 3.73 X 336 = 62,664 / 28.5 = 2199 rpm

This formula takes into account a transmission that is one to one ratio in high gear.

So my thought is that it is not as bad as I thought it would be. I actually thought it would be over 3000 RPM at 50 mph. It is however a lot of engine wear over time and a lot of gasoline. Austin wants his truck original so it is what it is. Shouldn't be slipping the clutch on take off!