If you want to accurately measure you MPG, do these steps:
(1) fill you tank to the max
(2) reset the odometer (tripometer)
(3) drive on the HWY (at 55 for comparison to other cars)
(4) drive using cruise control - to eliminate bad habit (VERY IMPORTANT)
(5) drive about 30 minute / 30 miles (longer = more accurate)
(6) now get gas again - fill it the MAX
(7) note the gallon just filled AND mile drive.
(8) calculate MPG = miles / gallon
You cannot use your GAS gauge to know if you finished using the gas. Also if you don't use the cruise you are putting in your driving habit, traffic, etc. on different days as variable. So you should do this at evening or night hours.
You can also use this technique whenever you think your car is gets better or worst MPG. It only cost you about 1 gallon of gas ($4)
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You'll get real world numbers. And this method give you the ACTUAL not theoretical numbers. So if you are comparing 2 cars, one or both may be running bad on gas and the above steps will show you.
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If you want to get great MPG, forget all those "SAVE GAS" scams. This was on NBC morning TV show. The difference is as much as 38% (their claim). There are very simple rules to save gas:
(1) drive slow - not too much past 55
(2) accelerate slowly
(3) don't brake unnecessarily
(4) use cruise control - whenever it is safe to do so.
Don't coast - it is NOT safe and it does NOT save gas.
The above really works. Using those simple rules I was able to drive from Boston to NYC (210.5 miles) on just 5.112 gallon (94 Civic). That's 41 MPG (94 Civic 187K miles). Actually I was speeding at 72 MPH so I could do even better at 55.