Can anyone figure this out. Have a 2006 LLY, 4l85E (overdrive trans?) , and putting a 8.5'' chevy rearend behind it in a car with 275 55 20'' tires. What gears do in need to get good mileage and performance (should not be a problem)!
Rearend Gear Calculations
- Thread starter max57
- Start date
I would think 3.42 or 3.73 would be a viable option(s) but Im far from understanding of the magic of ratios...
http://www.ringpinion.com/Calculators/Calc_GR.aspx
This may help
http://www.ringpinion.com/Calculators/Calc_GR.aspx
This may help
The .75 4th gear of the 85E is very close to the .71 5th gear of my 5 speed Allison in my 1/2 ton LLY. I have 3.42 gears with the same 275/55R20 tires in my truck. 3.42 are decent, and I have actually thought about going to a 3.23 gear to bring down my cruising RPM (around 2000 between 70 and 75). How much does your vehicle weigh? I think you'd be happy with either a 3.42 or a 3.23. If you want it a little better acceleration with that 2.48 1st gear, I'd say do the 3.42. If you want a little lower cruising RPM, I'd say do the 3.23. If I could find a set of 3.23s for the 8.5 IFS front diff in my truck I'd swap, that's the only thing that keeps me from doing it. Although my 3.10 first gear helps me get out of the hole better than your 2.48 will.
What are you doing for a converter? That will be a factor too, you could run a looser converter and still get away with the 3.23s.
I had 3.73 in my truck originally and it was too much.
FWIW, here are the ratios between the 5 speed Allison and the 4L85E:
Gear----Allison----4L85E
1--------3.10------2.48
2--------1.81------1.48
3--------1.41------1.00
4--------1.00------0.75
5--------0.71
What are you doing for a converter? That will be a factor too, you could run a looser converter and still get away with the 3.23s.
I had 3.73 in my truck originally and it was too much.
FWIW, here are the ratios between the 5 speed Allison and the 4L85E:
Gear----Allison----4L85E
1--------3.10------2.48
2--------1.81------1.48
3--------1.41------1.00
4--------1.00------0.75
5--------0.71
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I was thinking more at 3.00 or 2.90 gears to get some good fuel mileage. I am not sure on what it weighs yet. Going to a ford 9'' posi rear. I had it out in the driveway and it is single n wheel peel right now but shifted 1,2,3 in a heart beat and smoke was rolling for 200 ft before i let up with 2.93 gears. Factory 3600 lbs. so maybe 3900 lbs now.