Re: diesel mpg
1990 and newer is a HUGE margin. In that time there were several diesels. The most popular and maintenance friendly was the 7.3L powerstroke V8. The later 6.0 unit was unreliable and had a short shelf life at Ford (replaced with the new 6.4L). The 6.0L unit didn't get much better mileage than the 7.3L. If you plan to tow regularly then a low mileage 7.3L would be your best option. For everyday driving, again I recommend the 7.3L, but the 6.0 might be more appropriate for ever so slightly better mileage. The diesels I drove years ago (98' 7.3L) got on average 14mpg towing 6.5K and around 15-16mpg average not towing and with a light foot.
'11 Ram Quad Cab 4x4 SXT 5.7L HEMI (stock); 48,500m
'06 Pontiac G6 GT 3.5L 220hp; 64,000m
'12 Chrysler 200 Limited 3.6L 283hp; 28,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
Amsoil in all vehicles!