rear end gearing
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Elder User
Joined: Feb 2004
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rear end gearing
i was wondering if anyone would know what kind of a difference i'd see in a gearing change i'm looking at rightnow as far as acceleration is concerned. i have 26" tires with a 3.25 rear end gear and i'm looking at going up to 31" tires and lowering the gearing to 4.11. i've made a couple passes at a track and they were 14.94 and 14.933 and i want to lower that down a bit. the stock tire size is slightly over 29" (75 F-150) and the 3.25 is what it came with. i'm pullin 427hp at the fly and i'm just tryin to find a good gearing for about 31" tires.. on the rear anyway. i'll most likely keep 26" on the front because the trucks going to be lowered and lowered muuuuch more on the rear than the front. i just don't want to sacrifice mpg too much so I don't wanna be at redline at freeway speeds. with the stock tires and gearing i'd be doing 80 at 3000 rpm. oh and would going to 4.11's make towing any better if i wanted to do any? i've towed maybe 8k, trailer and bed cargo with the setup it has right now with 26" and 3.25 and it handled that like it was nothin'.
Senior User
Joined: Jul 2003
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From: North of Toronto, Ontario
Some quick math that I have figured out. Rear end ratio times the number of rev's per mile your tire requires equals engine RPM with a 1:1 final gear and no slipage, such as an auto with a slippy torque converter.
31" tires require 677 Rev's per mile so with these given ratios, your engine Rpm at 60 MPH will be:
3.25 2200
3.55 2403
3.75 2538
4.11 2782
4.25 2877
4.50 3046
4.75 3215
I don't know what tranny you have, these figures work with a manual tranny when you're in drive, which in most is fourth.
31" tires require 677 Rev's per mile so with these given ratios, your engine Rpm at 60 MPH will be:
3.25 2200
3.55 2403
3.75 2538
4.11 2782
4.25 2877
4.50 3046
4.75 3215
I don't know what tranny you have, these figures work with a manual tranny when you're in drive, which in most is fourth.
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Elder User
Joined: Feb 2004
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i have a c6 on it right now that died. so i still have to decide on what to do with that as well. whether or not to rebuild that or switch to a stick. but when everything was working right, the transmission was slipping into each gear at the track under WOT because it was old and stock with no mods for my engines power. i'm just tryin to figure out a good combo of everything. if i had an overdrive i would probably go maybe even a little lower than 4.11, but if i keep the c6 and get it rebuilt good with no slipping i think 4.11 is as far as i would want to go with some 31" tires. basically i want to try to keep rpm's the same for cruising but increase acceleration by spinning larger diameter tires the same quickness as i did the smaller ones. hopefully the gearing offsets the tire size the way i want it. and thanks for the numbers!
