Holy cow....

[Mikeandwendy74]

So I just took my 88 6.2 with a th400 that I rebuilt that shifts like brand new for a ride... come to find out that this truck I bought is geared waaaaaay low. I mean like anything over 55 and shes screamin'. Here's the question..how tough is it to swap the rear end gears for something that'll give me more top end without the rev and where can I find a set of these gears? I'm not looking to do alot of towing just a small boat and maybe some firewood for the campground. I have no problem with ripping the rear end apart. Any help is appreciated thanks.

[john8662]

2WD or 4WD?

First identify which rear differential you have.

Then, select a set of gears. If you're just wanting to get the RPM's down a bit and aren't going to tow heavy, I'd swap in a set of 3.42 gears.

Pull the cover and see what you have,

Sounds as though it could either be 4.10 or 4.56.

Randy's Ring and Pinion on the 'net is a good source for gears.

J

[More Power]

If you do a lot of towing & hauling, I'd keep the low gears, and install an OD auto tranny.

If you don't tow or haul, and are keen on the TH400, then 3.08 or 3.42 diff gears. The OD auto is still the best choice - in my opinion.

Jim

[Mikeandwendy74]

Oops its a full floater read end with the axles you remove to do the rear brakes. How can I tell which gears are in the rear end after I take off the cover is there a way to I.D them? I read somewhere in the forum that a the governor in the th400 help to protect the motor from blowin' but I tell ya its scary when I hit 60 I believe its a Dana Spicer D70-HD the truck is a 2 wheel drive c3500 1-ton.

[john8662]

When you remove the rear cover, you'll need a paint pen. Mark one tooth on the ring gear (big gear) and count all the teeth on the gear. Then do the same for the pinion gear (back of the housing, trucks front). Then Divide the ring gear tooth count by the pinion tooth count and you'll get your number. You will likely have to jack up the back of the truck to rotate the gears.

The engine has is own governor, it's in the injection pump, the engine will only turn ~4100 RPM max.

J

[wthif]

The engine has is own governor, it's in the injection pump, the engine will only turn ~4100 RPM max.

J

I thought it was only 3600 RPM. Does it just start defueling there or is it higher as you stated?

[gmctd]

Probably gonna be a 4.1 gearset if it's a Dana HD70, which usually resided in the 4wd 3500 chassis - a 3.25 gearset was available in the big corporate ff diff for the 2wd chassis with T400

EWC may have more info on that stuff.

[Robyn]

Most gear sets have the teethe count/ ratio stamped into the ring gear on the outer part that you can see with the cover off.

[Mikeandwendy74]

Thanks for the tips but I think i'm gonna go with the 700r4 sounds like an easier swap and personally i'd like my motor do be cruisin not screamin. I like the pulling gears I just want that extra gear.

[SoTxPollock]

TAKE THE EASY WAY OUT. Mark the tire where it sits on the ground, put a jack under the rear end and get it up off the ground. Turn the wheel one revolution and count how many times the driveshaft turns. If its a 4:10 the shaft should turn about 4 times while the wheel turns once. That should get you real close to knowing what is inside without haveing to start tearing down.

[JeepSJ]

TAKE THE EASY WAY OUT. Mark the tire where it sits on the ground, put a jack under the rear end and get it up off the ground. Turn the wheel one revolution and count how many times the driveshaft turns. If its a 4:10 the shaft should turn about 4 times while the wheel turns once. That should get you real close to knowing what is inside without haveing to start tearing down.

Not exactly...

If you have an open (non-posi or non-locker) diff, you jack up one tire then you will need to spin the tire two full revolutions. With a losi or locker, you will need to raise both tires and then you would only go one full revolution.

[Mikeandwendy74]

well I guess i'm kinda in a bind here. I didn't see the 55 mph scream coming because I bought the truck close to home, drove it 30 mph max knowing I was going to rebuild the tranny due to no reverse situation. I was kind of bummed out to take it out the other night after popping in the tranny and winding out on the highway. Does anyone know how to calculate the rpm's at 50-55 mph? After checking it has 4.10 gearset, 225 85r16 inch duallys and the turbo 400. Maybe its just me being used to gasoline engines...maybe i'm not turning the rpm's that I'm thinking. Might just be my nerves Holy cow!

[wthif]

After checking it has 4.10 gearset, 225 85r16 inch duallys and the turbo 400. Maybe its just me being used to gasoline engines...maybe i'm not turning the rpm's that I'm thinking. Might just be my nerves Holy cow!

I used to have a TH400 and would drive 65-70MPH with 4.10 gears and 31"-32" tire all the time. There was still enough RPMs left to peg out the speedo at 85MPH IIRC.

There is also an RPM calculator in the members area towards the bottom of the page.

[Mikeandwendy74]

I ran the calculator and came up with around 2700 rpm at 55. Sounds high to me but this is my first diesel....running at that rpm isnt going to blow this thing up?