Rear differential
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Freshman User
Joined: May 2003
Posts: 25
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Rear differential
I have a 1994 f150 automatic with the 300 i6 in it. its 2 wheel drive and has alot of ***** but i would like a little more top end and to put a limited slip differential in the rear. How do i go about finding out what gears are in the rear now and what differential to get? Also what dose anyone suggest for the gear ratios in the rear end to change them too? it is never used to haul anything so i am going for more of a sport/racing setup. Already have the K&N and puttin dual exhaust on after winter.
Post Fiend
Joined: Nov 2001
Posts: 14,947
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From: Topsfield, MA
Well the 300/6 is not a high/rev race motor like a V-8 so you want the RPMs to stay a little lower, therefore you need to keep the gear ratio pretty modest. If you're on the stock tires, I say 3.73s is the highest to go. That would put you in the 2100-2200ish RPM at 70 mph on the stock size tirs. You don't really want much higher than that. I might even say 3.55s if you do a lot of highway driving. Your truck either has 3.08s or 3.55s most likely, those are the most popular sizes. There is a tag on the back cover of the rear differential. It'll have two lines of numbers/letters...and should say something like 3 08 or 3 55.
Posting Guru
Joined: Jun 2001
Posts: 1,208
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From: Conyers
MustangGT221 He just said the truck already has plenty of gear, alittle too much for him as he wants a little more top speed. Asauppee, There are several ways to check your gear ratio, one would be to check the tag on the rearend, if you have a 3.55 rearend it'll read "3 55" and a bunch of other numbers. When you go to change gears, I wouldn't go down more than one or two sizes. (3.27 or 3.08 if you have a 3.55 ratio in the rear) If the gears have previously been changed or there's no tag, then you can just up a rear tire and put the tranny in neutral, make a mark on the driveshaft, and then turn the tire one full revolution while counting how many times the driveshaft turns.
