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Old 09-12-2019, 10:23 AM   #13 (permalink)
eco....something or other
Join Date: Jul 2009
Location: Colfax, WI
Posts: 705

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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Originally Posted by aardvarcus View Post
You can't talk rear end gear ratios without talking transmission gear ratios. The internet is telling me your transmission has 3.9:1 first and 0.8:1 fifth. Picking your rear end ratio to me is really about a trade-off in first and fifth, would you rather have a lower first or a taller fifth. For any other gear, you could just shift to alter the ratio but you can't go lower than first or higher than fifth.

In my experience I would prefer the 3.55 to the 4:10, as long as you don't think you will have a problem in first taking off loaded. With 3.9:1 first I don't think you will, but you know your engine/torque curve better than me. The fact that your donor vehicle is set up that way from the factory tells me it can't be that bad.

I would much rather have to downshift on the hills than to be downshifted all the time. To me a truck that drives around all the time in OD and never needs to downshift for steep hills is over-geared IMHO. It also seems you already own the matching 3.55 and would have to purchase the 4.10 front, so I would try it out before spending additional cash.

My brother said the same thing about already having the 3.55's. The donor zf trans has a 5.72 1st and .76 od with a 5.24 reverse. Either way I get the best of both ends, lol. 3.55 it is. I'll have 5 gears in the basement after the swap if I need them.

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