I have a 1981 d300 with a 360 and 4 spd (granny). the fuel mileage is poor and I wondered what trans would bolt in and give me a bit better mileage.

 

You already HAVE the "bolt-in" option, so the best thing you can do, is make sure your clutch plate is good, engine tuned up, diff fluid is changed, etc., so you get as much friction reduced as possible.

What's your terrain? ANY sort of hills will kill your mileage, regardless of transmission. Is it a DD in stop 'n' go traffic? Interstate cruiser? Do you haul anything IN it - or WITH it? What's your axle gear ratios? What transmission do you currently have? ALL these factors will affect mileage, and provide clues of where to look for savings.

 

I have an 1984 D250 with the 3.7 liter and I cannot get it to engage in any gear. I replaced the torque converter thinking that was the problem, but now I'm getting ready to replace the filter and fluid