Knowing that my car gets 29 mpg, I could just multiply that by the number of gallons I put in my car (to get the number of miles I should have gone) and then subtract that from the number of miles I actually went.

Then take that number and divide it by the MPG my car got between fill ups. Then take that number and multiply it by the current gas price to show how much money I saved.

Example:

29 mpg x 10 gallons = 290 miles
I drove 365 miles between fill ups
365 – 290 = 75
I got 36.5 mpg between fill ups
75 miles / 36.5 mpg = 2.05 gallons
2.05 gallons x $2.90 = $5.95

I don’t know if that makes more or less sense to do it that way.

When I fill up, I divide the number of miles my car has driven since the last fill up and divide it by the number of gallons I put into my car. Lets say I traveled 350 miles and put 10 gallons of gas in my car. I then got 35 mpg.

I know that the average gas mileage that my car had since I’ve owned it is 29 mpg, so I take the gas mileage for the latest fill up and subtract 29 from it.

In this case it’d be 35 – 29 = 6.

I then take that number and divide by 29 to get the percentage increase.

6/29 = about 21%

Let’s say I get I get 20% better gas mileage than usual, and the average price of gas is $3.00 per gallon. What I do is multiply $3.00 by .20 and come up with $.60. This number is what I’m “saving” per each gallon of gas.

I then take $.60 and multiply by the number of gallons I filled my gas tank with.

So, let’s say I added 10 gallons of gas, I multiply 10 by $.60, coming up with a savings of $6.00.

Does it seem like I’m calculating it correctly?