I don't own a Trooper turbo diesel or have I continuously driven one to figure out the mileage. But according to fueleconomy.gov the 86 Trooper turbo diesel gets 28 mpg city and 28 mpg highway. In that case it gives you the fuel mileage of the vehicle when it is brand new and I think it would be pretty hard to try to push for 30 mpg in a used Trooper turbo diesel. But I believe if you can get great air flow to the turbo diesel and open up the exhaust with a 2.5" straight pipe system then you can gain 2-3 mpg from those mods.