Lots of variables here based on the other mods you have or don't have. The higher the HP level the better the MPG if you have enough air to mix with the fuel to get full combustion, AKA full BTU use of the fuel then you'll be better on the highest HP level available. The other thing is the higher HP levels have a more responsive throttle, if you don't learn to drive with a much easier foot then you can have a little more of a "jack rabbit" starts and that can cause less MPG. So basically you need to make sure you have MAX air in and out, run on the highest HP level if you can control the off line throttle response. If you can't get that light throttle touch then drop back a HP level as it wil be less responsive and easier to control etc.
A forum community dedicated to Diesel engine owners and enthusiasts. Come join the discussion about performance, builds, ecodiesel, towing capacity, modifications, classifieds, troubleshooting, maintenance, and more!