I was asked by one of my web site visitors what the formula is for calculating how many miles-per-gallon she was getting on her car. Then it hit me that while I've been calculating my own cars' MPG for years it never dawned on me that some people didn't know how to do this.

So here's how to calculate your own car's MPG:

1. Fill up your fuel tank and write down the odometer reading from your speedometer.

2. Drive the car until you need to fill the tank again. Let it go down to about 1/4 of a tank or less.

3. Fill the tank again and record the odometer reading and the number of gallons it took to fill it. Be precise; record the gallons exactly including the portion of a gallon usually represented by a decimal in hundreds of a gallon. For example 15.78 gallons.

4. Subtract the previous odometer reading from the new one to determine how may miles you have driven between fill-ups.

5. Now simply divide the miles traveled by the number of gallons used. This will tell you how many miles-per-gallon you are getting.

6. For example let's say you went 356 miles between fill-ups, and it took 15.78 gallons to fill the tank. Well, 356 miles divided by 15.78 gallons equals 22.56. So you got 22.56 miles-per-gallon on that last tank of gas.

7. Check your miles-per-gallon over four or five fill-ups to get a good cross section of different driving conditions such as city, highway and so forth. Then take all your calculations and average them out (add them together and divide by the number of calculations) to get an overall picture of your gas mileage.

There you have it!