Thread: first 390 build

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I just did some research, I turn around 2500rpm at 55mph with a 33 tire I think I actually might have 3.73 4.10 4.11s, there ain't no slippage it shifts into 3rd so I might actually have a pretty low ratio

Originally Posted by riley1996

I just did some research, I turn around 2500rpm at 55mph with a 33 tire I think I actually might have 3.73 4.10 4.11s, there ain't no slippage it shifts into 3rd so I might actually have a pretty low ratio

Riley,
You're "around 2500rpm" is troubling, but let's run the numbers. You mentioned your 2500 @ 55mph again, and Denny assumed 28" tires, but you've said that your running 33's with those numbers. If we run the numbers, 1.0 final drive ratio, 33" OD rear tires, 55mph then you'll be spinning 2,554rpm with 4.56 gears; but you'll be at 2,302rpm with 4.11's. Not sure which "around" is closer for you, but if you were cheating a little above you've probably got 4.56 rear gears, of if you were below by a bit more margin then I'd say 4.11's. If you still had the 33" tires then it's a surety that you don't have 3.73's.

I agree that you need to get your hands around reality on this truck, and quit dealing with estimates, abouts, and guesses and start tossing out facts. It's not that hard to lift the rear wheels, chalk mark the tire and driveshaft and count the turns of the driveshaft with ten turns of the wheel.....

duplicate post

Originally Posted by rspears

Riley,
You're "around 2500rpm" is troubling, but let's run the numbers. You mentioned your 2500 @ 55mph again, and Denny assumed 28" tires, but you've said that your running 33's with those numbers. If we run the numbers, 1.0 final drive ratio, 33" OD rear tires, 55mph then you'll be spinning 2,554rpm with 4.56 gears; but you'll be at 2,302rpm with 4.11's. Not sure which "around" is closer for you, but if you were cheating a little above you've probably got 4.56 rear gears, of if you were below by a bit more margin then I'd say 4.11's. If you still had the 33" tires then it's a surety that you don't have 3.73's.

I agree that you need to get your hands around reality on this truck, and quit dealing with estimates, abouts, and guesses and start tossing out facts. It's not that hard to lift the rear wheels, chalk mark the tire and driveshaft and count the turns of the driveshaft with ten turns of the wheel.....

Roger and I are on the same page.
I plugged 2500, 55 and 33 into an online calculator and it spit out 4.46:1 gears.
Here's a list of available gears for a 9"...
Ford 9" Differential Parts | West Coast Differentials

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