Our 94 Burb has a 42 gallon tank so at 10 mpg with our 383 we can go 420 miles, rarely do that in a day. Love it, if we hit a lonely stretch in the middle of Nevada don't have to worry about running out of petrol, love having this extra protection. i quit worrying about gas mileage years ago, cause when I did I didn't enjoy RV'ing very much. Fill it, drive it and enjoy it. I usually don't look at the gas pump while its filling or afterwards. If you worry about mileage a lot, buy a Prius and stay in hotels. Big tanks are necessary for tow vehicles. We started out with a 1965 Chevy Van pulling 24' TT's with a 292 engine, had a 16 gallon tank so we planned our trips from gas station to gas station. actually still have the van and it still will tow.