Ok first of all, they are both right in certain aspects. Allow me to explain....
Internal combustion engine works on simple principle of you mix gasoline(in mist form) and air into a cyclinder, compress said mixture, ignite, and you have an internal explosion that forces the piston down, thus making car go. To produce more power, you have to increase the compression or increase the fuel/air mixture.
This is how turbos, superchargers, and NOS work(Superchargers and Turbos force more air into the cylinder using pressure, NOS splits the Nitrogen and O2 at high temperatures, thus allowing more O2 in the cylinder). They increase the amount of air in the cylinder during compression. The air itself does not increase the power, but the increase in air allows you to increase the amount of fuel you can put into the cylinder. Too much air and you get "lean", too much fuel and you are wasting fuel due to incomplete burnoff. You adjust the fuel mixture on carbs by "tuning" them, whereas EFI automatically adjusts this using computers(this is the reason that your car will run crappy if you ever disconnect the battery for extended period of time, it resets the computer and it takes a while for the computer to 'rebalance' your engine system to get to to peak efficiency).
So BZ is correct in that it takes a heck of a lot more gas to drive at 70 than it does at 30. Anyone who drives a car can figure that one out. Assuming identical gear ratio of 1:1 in say 4th gear, the gas pedal is much further down at 70 mph than it is at 40 mph. It take more gas to produce more power, which is why you never run your A/C when the fuel light is on. However, depending on how far the gas station was, he could have theoretically "coasted" easier at a higher speed than trying to coast in at 20 mph.