[david85]

It could get worse then that. First, the transmission also needs to be swapped since diesels (in fords at least) are a different bolt pattern (don't believe anyone that tells you a 460 is the same - it isn't). Also, if you are doing lots of short trips with a proportionally large amount of warmup time on your miles, you may actually get mileage comparable to a gasoline engine and in some rare cases slightly worse. The F150 I mentioned, got those numbers on a cross country trip over several consecutive fillups with average speed of 60-65 MPH if I remember right.

The 6.9/7.3 engines also require twice as much oil per drain interval. Other maintenance is generally lower, but I need 10.5L of oil with the powerstroke style filter I run (twice as large, and finer micron rating).

I was lucky with my F250 since it was already diesel and I use it mainly for highway driving. When I first swapped the rear end gears, the mileage improvement was such that they broke even in three months. The transmission swap took longer but did pay off after a couple years.