I just got back from a 3200 mile round trip from Minnesota to Big Bend National Park in Texas. Mileage ranged from 13 to 15.5 with an average of 14.4 mpg. My rv is a 1999 Leisure Travel Van based on a Dodge 3500 with a 5.9 liter engine. I drove between 55 and 60 mph and took mainly secondary roads.
My 78 Dodge with a 5.9 (carburetor though of course) might just get 14-15mpg then if I take it easy then since it lacks emissions equipment like newer fuel injected ones do. It has cruise control which hopefully still works. If not, I will see about fixing it. A 99 could use an OBD2 Scangauge to track mpg. I wish I could find something for my 78. Some/all Porsche cars from the early 80s had fuel economy gauges. Too bad you can't add one to other old vehicles easily. I wonder how accurate the Porsche MPG gauge was though. Looks like it probably estimated based on vacuum since it's analog and probably changes constantly like a tachometer and speedometer.
We have a 2008 PW Excel on a 2007 E-350 chassis with the V-10. In 30K miles we averaged about 12.5 driving at 65 on the freeway and some mountain driving, ~13.5 driving 50 on backroads. Get 8-10 in town if I'm carefull. Have gotten up to 15 if we're just putzing along on backroads at 40-45. Running Michelin XPS Ribs which 'seem' to have increased the mileage slightly from the original Hankooks. I also have a ScanGauge and it gives you a great feeling for mileage as you go.
aka 'Fossil Doc'
2008 Pleasure-Way TS2
I have been thinking about getting a scan MPG device. They seem like a useful device for both MPG tracking/improvement and for a whole collection of other readings including engine trouble codes. I also really like the fact that they can just be plugged into the ODB so they can be easily moved from vehicle to vehicle.