which ratio?
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Cross-Country
Joined: Aug 2005
Posts: 53
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which ratio?
i changed my 86 f-150 from an auto (aod) to a four speed. the truck has 4.11 gears in it and the tach says 2000rpms at 45mph. i am planning to change the differential with one that is higher geared. i want to be able to travel at highway speeds without terrible gas milage and high rpms. i have 31x10.50xr15's on the truck, i don't want to go bigger. the truck is high enough as it is. My question is, what gearing will allow me to go 70mph? i was thinking 3.55's or 3.50's. any advice appriciated. Thanx
Post Fiend
Joined: Jan 2004
Posts: 6,610
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From: Northwest, Arkansas
Thread moved to " Clutch,Transmission, Differential, Axle and Transfer Case" Forum. You should have more help here with your question. 
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"bigredman" Welcome to Ford Truck Enthusiasts. We are glad you chose to visit our site. In order to make it easier to navigate please read the following.
Please view the main forum page to find the best forum to ask any Technical questions
Also, below some of the titles of forums there is text that explains what the forum is either for or what shouldn't be posted in it.
Then ,Inside certain forum's there could be a Read First: thread or threads attached to the top.
Please read these threads before posting.
In Addition to the Tech Forums all user's can make their relationship closer to others by visiting & joining the State/Province Chapter that relates to their location.A complete list of Chapter's is located on the main forum page, near the bottom.
Once again...Welcome to the FTE Family
If you have mistakenly made a post you have 60 minutes to edit it.
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Postmaster
Joined: Jun 2003
Posts: 4,191
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From: WA
bigred, you should play around with a ratio calculator
http://www.dfw-tx4wd.org/gears.html
You plug in what tranny you have, tire size and gear ratios. You can see you RPMs at different speeds then.
If you are looking to stay around 2000 rpms at 70, you are probably looking at going to 3.0 gears. Alot depends on your 4th gear ratio.
Using that link I gave you, and assuming you have an NP435, with your 4.10's you should be at 2888rpm @ 65mph. With 3.09 gears its 2176rpms at 65.
You will give up power and acceleration going to a higher gear ratio (lower numerically)
http://www.dfw-tx4wd.org/gears.html
You plug in what tranny you have, tire size and gear ratios. You can see you RPMs at different speeds then.
If you are looking to stay around 2000 rpms at 70, you are probably looking at going to 3.0 gears. Alot depends on your 4th gear ratio.
Using that link I gave you, and assuming you have an NP435, with your 4.10's you should be at 2888rpm @ 65mph. With 3.09 gears its 2176rpms at 65.
You will give up power and acceleration going to a higher gear ratio (lower numerically)
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