[Dieselwrencher]

I'm not sure you'd get that high of mileage in a full size 67-72 or 73-87. I have worked on 2 different gmt400 trucks that had 4bt's in them. After we bumped the timing up and put different injectors in them and tweaked the injection pump, the owners said the trucks were getting 20mpg hwy. These were 3/4 ton 4x4 extended cad long beds. They ran ok but not what I'd want after I did a swap. In my opinion I'd rather start with a 5.9 and tune it to get optimal mileage and not sacrifice the power. The main reason is because a p pump 4bt is usually more money than a 6bt. By the time you start making decent power with a 4bt, you start sacrificing economy in a truck that weighs 4-4500 on up.

Sure, it has been done. It would be nice if someone like 744btcrew would chime in about his mileage. I know of a member here that had a 4bt in his 68 but later removed it in favor of a duramax and is much happier with his truck. Do you have a truck in mind to start with?