My build sheet that I requested from Chrysler tells me I have 3.55 gears, but my glovebox tells me I have 3.92.

Which one do I trust or is there a way for me to check? I know for a fact that I have the 8.25 rear - so did the 8.25 come with 3.92 gears as well or just 3.55?

Reason I ask is cause I want to go to 33" tires, but 3.55 wouldn't be a good idea with the auto tranny, so would 3.92 be alright or would it still be pushing it?

I know 4.10's are ideal but I dont want to shell out the 2 grand i've been quoted for it.

Thanks again.

 

Don't move up to 4.10 if you already have 3.92.

What transmission do you have? What are your RPMs at 70mph?

 

i went to a 4.1, BUT i drive ONLY in town. i do NOT recommend 4.1 gear if you do much hi way driving. the 4.1 got me more MPG cause it let me go to O.D. at 30 MPH, cruising at 30-40 i gained 3-4 MPG points. you might be able to readjust the linkage to the trans. to get it shift at lower MPH. but dont make a change that you cant return if needed. your gear can be checked by rolling the truck after marking the drive shaft.

 

I have the 42RE. And I'm at about 2200 rpms at 70 MPH.

Moe,

Most of my driving is city, but when I do take the highway its normally long drives 4+ hrs at a time. Don't care about mileage, just wanna make it easier on the tranny.

 

crawl under the truck. there will be a tag on your front housing or your rear housing. they will be matched so just find one.

 

Thanks sl2racer,

I remember finding a tag when doing my fluid change a few months ago.

So do you guys think 33" tires with 3.92 and mostly city driving will be alright?

Thanks again!

 

The BEST way to check your gear ratio is to pull the cover and read it off the ring gear. The next best way is to jack up the rear, rotate the wheels 10 turns, and count driveshaft revolutions. If you have an open diff, you can jack one tire and rotate it 20 turns instead. Divide the number of driveshaft revolutions by 10 and you have your ratio. You can also do it with one wheel rotation, but it's more accurate with 10.

I have 33s with 4.10 gears and they're perfect, even with lots of highway driving (and often well above the speed limit). I don't think I'd want a taller ratio with my truck, but you can probably get away with 3.92 because you have an automatic.

 

Thanks Tom.

I just had the cover off recently so I don't wanna take it off again so ill try your method by rotating the tires. If I do indeed have 3.92 in it I'll be putting on 33s

 

3.92

 

I crawled under there and checked the tag and it says "3.9".

That could also mean by engine

Time to find some tires in my area. GF is gonna be pissed. Cheers guys.

PS - Found my brake fluid leak, driver side rear tire is the culprit. Yet to find out exactly where yet though.