[74 shortbed 406]

I had an 87 k10 a couple years back that cam with 3.08. I broke those axles and swapped in a set of axles with 3.42 and my mileage went up to about 16-17 from 13-14. My motor was lugging and the 3.42 help it out a little. Also check the tire sizes, I think factory tires were 235/75/15, get a lighter tire for less rolling resistance. Grease all u joints, change fluids and gear oils, if you have a bug shield take it off and you will get close to 1 mpg gain.