[quote author=NaplesBill link=topic=34339.msg1120983#msg1120983 date=1167173876]
The easiest way to calculate your mileage is to fill up and reset your trip meter. When you fill up again you divide the number of gallons it took to fill up into the mileage on the trip meter. This will give you a valid calculation of miles driven and gallons of gas used.
I second what NaplesBill said
. Unfortunately, the problem is, it is not the easiest way, and too many are looking for the easy (trip computer) answer. In fact it has been stated so many times, it should be common sense, so I neglected to add that to my post. :bash:
I would add to that by saying it is the only
accurate way to figure your mpg. And you must do it over the course of several tankfuls to get an accurate reading, and it will vary according to outside temperature, type of fuel (oxygenated or non), traffic, HVAC usage, load factor in the car, driving style, etc, etc, etc.
A good idea is to keep one of those small notebooks in your car (5 inches X 2.5 inches or so). Each time you fill up, note the miles driven, and gallons needed to fill. Then do the math, and you will get a long term idea of your actual mileage. I keep one in my glove compartment, marked with three columns: Miles, Gallons and MPG. I always fill my cars, never just $5 or $10. Could actually tell you the individual mileage for every tankful of gas I've put in my cars since 1981 (my first car bought new).
Use the trip computer (Trip A, or Trip B, to track your mileage per tankful), and reset it when you fill up. Once you do it for a while, it will become second nature. It also gives you a great idea of how your car is running over the long haul. If your mileage starts to fall, you know immediately, and can address the problem.
It also helps if you have this sort of documentation when you go to the dealer for MPG concerns. Much more persuasive than just walking in and saying "I'm only getting XXX MPG, and I don't think my car is running right."
Hope this helps, and good luck!