Trying to decide whether to replace noisy 3.1L with rebuilt 3.1 or goodwrench 3.4, in 91 4wd pup. Axle ratio is 4.3:1 and I run 31 or 33 inch tires. 3 in body lift and 2 inch suspension lift. 4.3L tbi and intake bored to match, are installed, plus super turbo muffler.

Read nomans post about improved mpg with 3.4, in 90 trooper. Amazing

Will I lose mpg or gain with 3.4? Best mpg with 3.1 has been 18 hiway.