Fords name for their factory limited slip differential is the Trac-Lok, it's a clutch loaded unit that can deliver near locker performance when setup correctly but is totally smooth and silent in operation compared to a real locker. They're relatively cheap to buy new and plentiful in junkyards too.

 

Well thats what I thought it might be (posi-traction). Guess it has multiple name now. Well, its off to gas up the boat @ truck, I have a bass tournament tomorrow and probably in the rain. Everyone here have a great New Year. I report back after I get that stuff installed.

 

Check out the eaton tru trac , its more expensive but it dosn't have clutches to were out & locks much better, and is very smooth , once its in thats it your done ...Lew

 

I agree with going with the 4.10 gears since you have 31s. Also get a traction lock or similar limited slip diff. It can be rebuilt. I have one and think its great. Auburn also makes a good one.

I won't ever own a true locker again. I don't think its a good choice for a daily driver. Its more suitable for a mud truck or drag vehicle.

 

i love my locker though. its fun. but i definatly understand what you mean its not suitable for a dd. i can understand that thought. but like lew said get the tru trac. no clutchs to wear out and brake.

 

When you put in lower gears, do you have to change anything for the speedometer?

 

depends on what year and what tranny. on my 92 with a E4od the sensor is in the rear end and automatically adjust for the gear swap. like i went from 3.55's to 4.10's. and i had to change nothing. and the speedo is still dead on accurate. now on older C6's there is a speedo gear in the tailshaft of the tranny and you have to change that cause it changes the Rpms of the output of the tranny and i think on the new ones like after 86 or so there are 2 spots. the rearend and the tailshaft i think probably wrong. i believe the M5od is the same setup as the C6 but may be wrong bout even the C6 setup.

 

Your truck most likely has a cable driven speedo off the transmission so you will need to recalibrate, it's done by changing the driven gear on the trans end of the cable.

 

The 92 and up F-trucks use electronic speed detection on the ring gear so the computer couldn't care less what gear ratio the rear axle has nor what gear you're in transmission wise - the only thing that will confuse it's ability to detect speed is tire diameter. Larger tires give a false reading of "slower" than actual, smaller diameter tires give a false reading of "faster" than actual.

The cluster is also programmable so you can change your tire diameter five times before the cluster won't allow you to change any longer.

Since the original poster has an 87, his speedo is connected via cable to the transmission, so rear end ratio changes and tire diameter changes impact it's accurancy.

I really don't know why you wouldn't just select 3rd instead of overdrive. It's obviously easier to do than changing out the ring and pinion. If you tow constantly, then 4.11 gears would be the way to go. But for a one-off or occasional/seasonal towing, I'd just avoid overdrive.

 

3.90's are an option for the 8.8 axle, if you're interested.

 

I didn't know that 3.90 gears were an option for the 8.8. I think 3.89 was the ratio for the 9 inch and maybe the 8 inch. I have never heard of any one with an 8.8 and that ratio or seen it listed either.

 

Yeah, it's not something you see often. A few late model Mustang guys run them. Pro 5.0, Richmond Gear, Motive should all list a 3.89 or 3.90 for the 8.8.

Now if only they made a matching gear set for the front D44...

 

How do you know what speedo gear to get? Like in my case, I have the M5OD, 3.08's, and 31-10.50-15; if I went to a 4.10 gear, what speedo gear would I need, and where would I buy it?