I you want the best MPG use road tyres, not A/T tyres. You will also get better ride quality, lower noise and much better on-road handling...They will probably last longer too.off topic but maybe interesting, my 02 kj has done better with firestone destinations as far as mpg than others, has anyone found better?
UK spec 2.8 CRD Sport's came with the Mercedes NSG370 6 speed manual gearbox...With a 6 speed the KJ is like a dragster away from the lights in 1st, but you can get up to 38mpg in 6th.A Diesel would be awesome, but before installing it unless you are overseas you will have difficulty passing it through inspection. I think you have to go with a motor made in your year or later.
On the flip side, it would be very expensive to do also. The computer work would be the biggest pain.
What I would think would be neat mod would be ridding the 4spd trans and replacing with a 5spd. I think the computer reprogramming would be a nightmare.