Gears and RPM's
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Gears and RPM's
I have a 1987 F250 with a 300 4.9L and auto C6. Just curious what rpm I would be running with 4.10 gears on the highway say 55-65 mph. I don't ever need to go faster and rarely drive on the highways any long distance and I pull a trailer or load almost daily.
Post Fiend
Joined: Nov 2002
Posts: 9,987
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From: Memphis, TN
Your RPM is also dependent on your EXACT tire size, so get ALL the numbers off the sides of the rear tires. It should be something like LT235/75R15 or LT31x10.50R15.
Once you have the tire make, model, & size, you can probably look it up on TireRack.com and find the Revs per Mile for that particular tire. This page shows the specs of Bridgestone Dueler ATs.
Then the formula is:
(Revs/Mile) x (Miles/Hour) x 1 Hour/60 Minutes = Revs/Min of the tires
At 60 MPH, the tires' speed is the same as their Revs per Mile because 60 MPH = 1 Mile per Minute.
After you know how fast the tires turn at a given speed, you just multiply by the rear ratio to find the driveshaft speed, which is the same as the engine when you're in 3rd gear (Drive) & Hi range.
So the calculation would be something like:
680 (Rev/Mi for a 31x10.50R15) x 60 (Mi/Hr) / 60 = 680 RPM (TIRE speed)
680 x 4.10 = 2788 RPM (DRIVESHAFT speed) If your t-case ratio is 1:1 and the transmission is also in the 1:1 gear, then the engine RPM is the same as the driveshaft.
Once you have the tire make, model, & size, you can probably look it up on TireRack.com and find the Revs per Mile for that particular tire. This page shows the specs of Bridgestone Dueler ATs.
Then the formula is:
(Revs/Mile) x (Miles/Hour) x 1 Hour/60 Minutes = Revs/Min of the tires
At 60 MPH, the tires' speed is the same as their Revs per Mile because 60 MPH = 1 Mile per Minute.
After you know how fast the tires turn at a given speed, you just multiply by the rear ratio to find the driveshaft speed, which is the same as the engine when you're in 3rd gear (Drive) & Hi range.
So the calculation would be something like:
680 (Rev/Mi for a 31x10.50R15) x 60 (Mi/Hr) / 60 = 680 RPM (TIRE speed)
680 x 4.10 = 2788 RPM (DRIVESHAFT speed) If your t-case ratio is 1:1 and the transmission is also in the 1:1 gear, then the engine RPM is the same as the driveshaft.
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