Outside of the initial truck config, here are some factors:
- Diesel's best advantage over gas is at partial (light) loads, the gasser runs at 13:1-15:1 fuel mix to light off the fuel with a spark, a diesel can run anywhere from 13:1 to 100:1 fuel mix, under max loading the fuel mileage is about equal for gas vs. diesel.
- Every cold start will reset your average about 1-2 mpg lower
- Wind direction is a huge factor with a diesel, wind at a slight angle can be worse than a head wind, a tail wind will have a huge effect, driving in Texas I've never had a tailwind.
- Altitude in general will give you better mileage, wind resistance falls off by the square with density, I've seen 2-3 better mpg at 5000 feet.
- Speed is huge (it's wind speed), resistance goes up by the square, from 55 to 65 to 75 wind resistance almost doubles with every 10 mph.
- I've found hills to sometimes give better mileage. 18 mpg on flat vs. 13 mpg up hill and 30-100 mpg down hill, do the averages.
- The stock speedometer always reads high by 5% so the gauge and your hand calculations will be off.
- bigger tires will usually drop mileage.
- A lift will drop mileage.
- Dropping the tailgate will drop mileage. Covering the bed will help. Covering only the rear 1/3 of the bed will help more.
- Weight will drop mileage for hills and stop-start, but it is not a big effect at steady speeds.
- The best beer is Pilsner Urquell.