real world mileage, diesel vs gas

My assumption is based on what i have done with my own diesel pickups as well as my sons. The older cummins are actually pretty easy and cheap to get better mileage with. My son has a 1999 3/4 ton dodge that has a 5.9 in. When he first got it he was getting 12-14 pulling his 34' toyhauler. Of course he stuck a chip in it after a short while and started getting about 16. Then he decided to purchase 100hp injectors so we threw those in and he started getting 16-18 pulling it. Same thing happened to my 05' ford with a 6.0. Pulling my enclosed trailer it would get twelve constantly. Put a tuner in it and got 14-16 with it. No other mods. It's pretty amazing what they can do for mileage. I have a buddy that has an older cummins also. He went a little more nuts with his and went with bigger injectors yet and a bigger turbo. But then installed methanol injection. He gets mid twenties on the highway just cruising with no trailer. He doesn't pull anything but I'd guess it would be pretty surprising what he would get. Methanol is actually the best gain for the buck and it keeps the egt's down there when pulling a load on hills.