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Old 09-09-2019, 10:43 PM   #1 (permalink)
IsaacCarlson
eco....something or other
 
Join Date: Jul 2009
Location: Colfax, WI
Posts: 707

wood hauler - '91 Ford F-250
Team Pontiac
90 day: 18.97 mpg (US)

Rav - '06 Toyota Rav4 Base
90 day: 26.52 mpg (US)
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Exclamation Help with gearing

I'm having a hard time deciding what to do.

I am finally making my truck 4wd, but the donor has 3.55 gears and I have 4.10's after getting a new axle several years ago.(4.10 was the only thing available) Do I swap in the 3.55 gears or buy a matching 4.10 for the new front end?

I currently have the m5odr2 which has a fairly high low end and not much overdrive. The 4.10 helped with the low end, but hurt me on the top end. It pulls like a dream and it's nice to not down shift for most hills when loaded, but it just feels like it's screaming along at cruise.

The donor has the zf5 which has a deeper low end and higher top hole. I think it would be fine with the 3.55 gears because it has the deeper gears on the bottom to make up the difference. The extra overdrive would be awesome too.

It's going to be ~20% reduction in wheel torque in the top hole with the 3.55/zf combo by my math. About 14% with the rear end and another 5% with the overdrive step. I'd be shifting down on hills for sure.

I normally weigh ~12,000 lbs loaded, so that lost torque will be noticeable.
Shifting down to 4th for hills will put my rpm's higher than they would have been with the 4.10 in 5th gear.
M5od/4.10 is 2100 in 5th
Zf5/3.55 is 2270 in 4th

I'd like to get my rpm's down and get a few mpg back, but I don't want to lose my grade pulling ability.

What would you guys do? Any advice? Feel free to toss ideas around. I'm not buying two new 3.73 gear sets, so that is out.


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1991 F-250:
4.9L, Mazda 5 speed, 4.10 10.25" rear
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