MPG - general case
Apologies is this has been done to too many deaths ...
Brake Horsepower Specific Fuel Consumption (BHFSC) is expressed in pounds per hour per horsepower. We can convert this to metric, but the same principles apply.
It's going to be *around* 0.4 lb/hr/hp ... a little better for some of the newer technologies, and a little worse for some of the older (carburetors). It's going to be a little better for diesel and a little worse for most gasoline (maybe direct injection?). but it's most likely going to be in the .35 to .45 range, with most of the action around 0.4. I don't think you'll find yours without some quality time on a dyno at different speeds and power settings, and the OEM isn't going to tell you directly (it's going to be different at different places on a torque map, and that's a competitive disadvantage. You can guess at where your sweet spot is by figuring out what your fuel burn is at 60 or 65MPH, but that will take some patience.
A gallon of gasoline is 6 pounds, a gallon of diesel is 7, which changes nothing on the "pounds per hour" scale, but does good things on the "gallons per hour" scale.
A car getting 30MPG at 60 MPH is burning 2 gallons per hour. If gasoline, this is 12 pounds per hour, if diesel, this is 14 pounds per hour. (Translate it as 1/60 hours per mile and 1/30 gallons per mile, then divide to get gallons per hour, and multiply to get pounds per hour). We can turn that around and say how many hp it takes to go that speed. if we take 12 pounds per hour and divide by 0.4 we'll get about 30hp to move that car along at 60 MPH. That sounds about right. That 350hp sports car spends most of its life at 30-40hp, same as that VW Beetle your parents had.
What eats horsepower ... Three main things:
1) aerodynamic drag
2) parasitic losses
3) The driver
4) rolling resistance
Drag is enormously important - there is a number called the "equivalent flat plate surface area" which is basically the coefficient of drag times the actual frontal area. If you've pulled a square camper and an Airstream, you have a firsthand understanding of this. The Airstream isn't perfect, but it's much less bad than most everything else. Pressure, volume, and temperature come into play here - dense air is draggier than thin air.
Parasitic losses are going to be pretty constant ... However, run the HVAC fan on full with extra lights and the air conditioner clutch and engine cooling fan at full tilt, and you'll chew up some tens of horsepower. You'll see it as a loss of MPG, but ... Several years ago I spent a little time with Metro Dade Transit Authority, and was told that the alternator loads alone on a city bus (hot humid day, raining, after dark, articulated bus) were something like 70HP.
Truck fleets exist to make a profit (mostly), and are moving to single rear wheels to save fuel (there may be other reasons). It's a small amount, but it has an ROI for them. Similarly, concrete has a lower rolling resistance than tarmac, etc.
The single biggest factor is the driver, however. There have been some articles in the trade magazines (like Heavy Duty Trucking, Fleet Owner, stuff like that) stating that a fleet might average 6.5 MPG on their class 8 trucks, but some drivers are getting in the high 7's - like 15% difference. You see the same thing here - one driver will report 9MPG and another 11 or 12 (or 12 to 14.5 for diesel). This is normal. If you need more fingers than one hand's worth to count the number of times you've buried the throttle in the last year, you're probably getting better than average fuel economy. On the other hand, if every stop light is a drag race and your cruise has never seen a set speed under 70, probably PROBABLY, you're not getting the same fuel economy as that other guy.