CarTalk.com Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Rear end differential

on a 1997 jimmy 4wd with a 3.73 locking rear differential.would it be cost effective to change to a 3.08 gear

You can get a much better idea on whether it will be cost effective, if you get the following information:

1: Approximate price to switch the differential (get quotes)
2: The average number of miles you drive/year.
3: The current miles/gallon your jimmy is getting.
4: Cost of fuel/gallon.

IE, you first need to know your annual fuel costs today, plus the cost to replace the differential.

Then the only unknown variable will then be how much the new differential will improve your mileage (if any), and you’ll be able to figure out if it’s financially worth it.

So the question to this forum is: Will changing the differential improve gas mileage on my 4wd jimmy, and if so, by how much.

My sense is it will help very little (1-2 mpg tops under ideal conditions), and that you may never recoup the costs.

And remember on a 4x4 you must change both front and rear ring gear & pinion sets. $ cha-ching $

Probably not due to the need to change both as Ken pointed out. You also have to consider two other things. The jimmy has a 3.73 for a reason, tow capacity. If you don’t tow with it, fine, put in something even more fuel-friendly. The other consideration is where do you drive. Often a gear change will not make much of a difference around town because the transmission will shift into higher gears later with a higher RAR. On highway cruises it can make a big difference.

Let me just add that unless you have a highly experienced rear end specialist do the job, you will wind up with a howling diff. (or two)

If you still want the change done be prepared to expect a very high bill. Each rear end diff change averages $1k each.

Figure about $1200 or so for parts/labor. Unless you put a couple hundred thousand miles on the truck, it’s unlikely you’ll recover the inital costs. As an add bonus your truck will accelerate slower too. Most of the time when someone changes out a ring & pinion it’s done to increase performance. My mustang has a 4.10 rear end (stock is 3.27) and my Bronco has 4.56 gears (Stock is 3.55, but it is fitted with larger than stock tires, so it’s effectively something like 3.89 overall).

Short answer… No way.