You'll find your mileage monitor will be significantly wrong. Be sure you get the latest computer flash as you'll get better mileage then the US average 11 and you'll be able to "monitor" the regen cycles that are performed by the ecm to prolong the life of your DPF (diesel particulate filter).
'99 F350 Crew Cab 4x4 Lariat Off-Road V10
Vortech supercharged with aftercooler, 4" Pro-comp, 35" Nitto Terra-grapplers, Headers, Flowmaster cat-back, March underdrives, Diablosport predator tuner.
'07 Salem T26BH
If you want to buy one of these just take the cash, wad it up and throw it into the garbage.