How to calculate your car's MPG?

I used to drive a 2005 Hyundai Sonata GLS. I do love my car. It has given me no trouble and I have only done scheduled maintenance and regular oil changes. Given that I drive a lot, I try to keep an eye on costs. I care about how much I spend on gas since that is a very important component of my expenditures on the car.

[ Silver 2005 Hyundai Sonata by the lake (winter time) ] Incidentally, I do not drive 30 miles to save five cents on a gallon of gas but when I do travel out of town, I make sure to fill up at various gas stations that seem to consistently have gas prices almost 25 cents lower than in Ithaca.

Measuring gas mileage for all cars of a particular brand is pretty difficult as driver habits, the overall condition of the car, where you drive, how fast you drive, the weather etc all affect how many miles you can get out of a given gallon of gas. So, the EPA ratings tend not to reflect the real life experience of a lot of people. For your information, the EPA ratings on my car are 17 mpg and 25 mpg for city and highway driving, respectively.

What is the right way to measure the MPG for your car? The most important part of measuring the MPG is to always fill up and record how many miles you drove between fill-ups. It is best to try to get gas at the same station and the same pump as the point at which the pump cuts off the flow of gas varies from pump to pump. I am not this careful. However, I do stick with the rule of never topping off.

I try to fill up when I have about 1/4 of a tank. In New Jersey (where self-service is out of the question), you have to make sure the attendant does not try to top off.

[ Figure 1: Gas Mileage between Fill-Ups ] Figure 1 shows the MPG I get between fill-ups. One thing you should notice is that sometimes the time interval between fill-ups is very short and sometimes much longer. You might also notice that when the time interval is long, the MPG falls. Why? Because highway driving consumes less gas per mile traveled but you consume gas more quickly on the highway as you presumably are driving longer distances. On the other hand, when I am driving around Ithaca, I travel shorter distances, consume less gas but get awful gas mileage. That is because Ithaca is hilly, there is always some stupid construction going on somewhere, you have to wait at lights and stop signs etc.

I have been keeping track of these data for about 15 months now. Given my driving habits and all the other conditions pertinent to me, how many miles do I get per gallon?

Taking the average of all data points in Figure 1 is not the correct way to calculate the long run fuel efficiency of a car. This is because the number of miles driven between each fill up is different. The right way is to divide all miles driven by all the gas consumed over this period of time. That comes to about 23.7 MPG gallon with a corresponding cost of about 11.3 cents per gallon. These data are summarized in the charts below:

[ Figure 2: Cumulative Miles per Gallon ]
[ Figure 3: Cumulative gas cost per mile ]