You should test drive a dodge with the cummings in it, they get even better mpg than my tacoma, but i allso have a heave foot and 285;s on it.
Scan gauge does not need to learn a vehicle it reads the fuel rate which is a function of the factory computer and then calculates the mileage on the fly based on the speed you are driving. how can that have any "learning" involved as long as it can read fuel rate and your speedo is not out it should be pretty godamn close. The truck i test drove had stock sized tires on so the comptuers speed reading should be within 5% or soi very seriously doubt you were getting ~18mpg in that ford (not just because i hate them). the problem lies in your scangauge. sure it may SAY 18mpg but the deal is the scangauge takes time to learn and calibrate itself for each vehicle (like 500-1000 miles). mine always read higher than what i was actually getting for the first 1200 miles, so i think you were really getting somewhere around 15-16 at best...
but honestly dude if you want a diesel get a dodge 2500 or 3500. my friend's got a 2500 with the 5.9 cummins in it. he gets 27mpg going 80 down the interstate!