Advice on picking rear end gears

Question:

I purchased a new Chevy 4x4 truck in 1977. It had a 350 cid engine and a 5speed manual transmission and 3.07 or 3.08:1 gears in the rear end. I pulled a heavy camper trailer with it without experiencing any problems.

I currently own a 1991 Chevy truck with a 350 cid engine, 5-speed manual transmission, and 3.42:1 rear gears. The gas mileage is around 17-19 mpg when I baby it.

I’m in the process of ordering a new GMC or Chevy truck with a 5.3-liter V-8 and 6-speed automatic transmission in it and I was going to get the 3.08:1 gears in the rear end, hoping to get a little better gas mileage. Do you think that I would be better off going with the 3.42 gears or the 3.08? I do occasionally pull a 16-foot car hauler with it when I pick up a Chevelle. Will I get that much better gas mileage with the 3.08 gears instead of the 3.42?

Answer:

The EPA fuel economy rating for a 2010 Chevy Silverado with a 3.08:1 differential is 15/21 MPG and with a 3.42:1 differential it’s 14/18 MPG.

While actual fuel use will, of course, vary in real life the difference in the EPA rating is likely pretty close to the difference you’d experience. So, the benefit is about 1-3 MPG more with the 3.08:1 gear ratio.

What’s the downside? I don’t see one. Given that you tow only infrequently and given that the 5.3-liter engine and 6speed automatic combination can easily handle the load with a 3.08:1 differential I’d opt for the more fuel efficient 3.08:1. The difference of 1-3 MPG may not sound like much but, depending on your driving habits, it will represent around a 10% increase in mileage and that, of course, translates to about a 10% savings in fuel cost. That will add up to quite a few shekels over the life of the truck.