Plus all gauges are potentially going to be off. Closest way to getting true MPG is filling all the way up and resetting a trip. Go 100 miles, heck even 50 if you want and refill. See how many gallons it takes to fill her all the way up again and divide the miles by that. Doing it that way (old fashion way) is a good way of testing gears, ridding habits and such. You don't need to run through a whole tank of gas to work out how many miles you are getting per gallon. Of course that is assuming that the odometer actual reads 1 true mile for a mile traveled.
I can't be that exact in filling, but the farther I go the more precise my calculations. As an example, 50 miles and 1.25 gallons would be 40 mpg. Dribble 4 more ounces in, and my mileage drops to 39. Make that 200 miles though, and a few ounces matters less -- a lot less. But it still matters.
I think that's why my on-board computer is so far off on many tanks. It has no way of knowing to what point I'll fill it. I checked it just out of curiosity on a trip one time over ten tanks. I filled it, made note of the odometer, and zeroed the on-board calculations. Then I kept track of the gallons over the next ten tanks, totaled them, and calculated the mileage over something like 1800 miles and 4 days of varying conditions, and it was really quite close to the on-board computer -- within a fifth as I remember.
Yeah, I'm going to have to go from mile to mile marker on the highway one day to check it out.
Bear in mind that the mile markers over one mile could be off quite a lot. They won't, for instance, put one in the middle of an exit lane
Again, over some distance they're more accurate. I can show you a place near my home where two mile markers are about .8 of a mile apart. But the next two are closer to 1.2. Both are wrong, but combined they're almost perfect over two miles. I always check the odometer over a distance in a few different places. Taken as a percentage mile markers over ten miles are probably a lot more accurate than over one mile.
Oh, I know. But numbers are kind of a thing with me.