Has anyone had expierience with changing the final drive gear ratio from 3.90 to 3.70? I have a pushbutton 4WD 5spd in my 87 gl-10 turbo that has a 3.70 output. I would like to install a D/R 5spd, I found one with the 3.90 ratio. Can I just swap out the final gears on these transmissions? Will everything interchange? I would like to keep the 3.70's for better mileage, I also have heard the axels may be larger on the 3.70 But I haven't compared them. I don't know how hard these trannys are to work on. Would I be better off waiting untill I come across a 3.70 D/R?.......Roger

Would I be better off waiting untill I come across a 3.70 D/R?.......Roger

 

Yes

 

What you a looking at is an almost complete tear-down of the transmission to do this.

I can't offer all the answers you're looking for, but I can offer this:

 

I doubt you'll notice much difference mileage-wise between the 3.9 final drive and 3.7...the 3.7 will be turning fewer rpm's, but fewer rpms means more load....this question has been discussed, and the agreement was, that with an EA82 motor, the differences are going to be slim, with stock tires anyway (whole different world once us lifted guys get into almost 30" tires)

 

part-time D/R boxes are all 3.9. RX's had full-time D/R with 3.7, but the lo range reduction is quite a bit less than the part time

 

trannies are very complicated, lots of delicate adjustments to insure that all the gears mesh just right. personally, I won't open up the tranny. IMO, too much risk of majorly screwing something up!

I guess I would probably be better off changing the Trans. and rear diff. and go with the 3.90. I dont want full time 4WD. So my question now is will I also have to change the axels? Are they larger on the 3.70 turbo trans and rear diff?

I doubt you'll notice much difference mileage-wise between the 3.9 final drive and 3.7..

 

there's a substantial difference in the XT6...sort of.

 

XT6 automatic AWD 3.7 final drive - 28.7mpg max highway mileage

XT6 manual AWD 3.9 final drive - 27 mpg max highway mileage

 

note the auto getting higher mileage than the manual, due to the better gearing for highway cruising.

 

*edit* as dxrflyboy mentions below this is due to overall gearing in the trans and final drive (small percentage).

 

i've owned quite a few of these and done lots of highway driving. the automatic runs about 3,000 rpm's at the same speed the manual is running 4,000 rpms. worth it for the noise factor as well. cruising at 4,000 rpms is loud. lots of power if needed, but noisey.

Do the math. Gearing down to 3.9 from 3.7 is a 5% increase. All else being equal, if the engine is spinning at 3000 with the 3.7 gear, it will increase to 3150 with the 3.9. This is a small difference at cruising speed. It makes much more difference in the lower gears, giving you more torque at lower vehicle speeds for better acceleration and hill climbing.

copy that. the gearing within the transmission is to be paid more attention to for milage considerations. so he should be concerned about swapping his 5th gear for the best mileage and leave the rest as is if he wanted to get that far into the trans. or look for a trans with the best 5th gear ratio for mileage.

you guys are making this way over complacated. heres is how you do it...

 

pull your 3.9:1 D/R and diff...

i will pull my 3.7:1 D/R trans and diff...

no put it in your car or truck and drive north...

and i will do the same but drive south...

we now swap tran's and diff's...

revise process...

 

done.

 

-=Suberdave=-

 

PM me if interested.

Wow Suberdave, that would really make it easy for me. I will PM you. Thanks everyone for the replies. This is the best forum I have ever found, I've learned a lot about soobs the last 3 months.