In my experience with the Dodge trucks and Cummins 5.9 12V diesel the payback was fairly quick, if you consider that I was able to put 300,000 on one truck with zero major repairs. Sold it still running fine so I don't know how much further it went. I did have to replace the clutch once, both front wheel bearing cartridges as it was a 4x4 and of course the serpentine belt and adjust the valves when the spirit moved me. Considering the longevity and the savings over time from the better fuel mileage I imagine breakeven was probably around year four. Since this was a 3500 dually if you compared a similar gas engine truck I would guess the gas rig would get about ten MPG average if you're honest. I got 17 to 19 depending on load, and for pure city driving 15. Newer engines I understand are nowhere near as efficient and trouble free.