Re: Swapping in a 258 in place of a 304
All good info so far. The '80 CJ5 should not be computerized. That didn't start until '82 I believe and wiring should be identical for both. Some wire routing and length changing might be necessary. As Dirtrod said the RPM's at road speed won't change. The 3spd and 4spd have the same gear ratio in high gear. Try driving that 304 like you have an egg under the throttle pedal and see if it helps. The 258 would be better, but to what degree? At highway speed it's really the aerodynamics that are killing the mileage and the 258 may take a little more throttle than the 304 does.
'75 CJ5,258 w/Howell EFI,T18a,4.27's,33's,On-board air,Warn 8274