There are lots of reasons why you want to measure your car's gas mileage – to make sure it's running right, to measure the effects of any techniques or gadgets you're using to improve your gas mileage, etc. – but exactly how do you measure it? It is very important that you measure your gas mileage correctly and consistently, so that you know you are getting results you can trust.
The mathematics behind gas mileage are as simple as can be: distance driven divided by fuel consumed equals fuel economy. In the United States, that means miles per gallon, but the same principle works for kilometers per liter. It also works the same way whether you are burning gasoline or diesel, petroleum-based fuel, ethanol, or biodiesel. But there are two challenges:
1. How do you determine exactly how much fuel you are using, and whether a given gallon of gas should be included in the measurement for a given mile?
2. How do you keep track of gas mileage over a long period of time without missing any gallons or miles in your overall calculations?
There have been a few articles published on that first point which, in my opinion, make the process unduly complicated. Here's the easy way. Start with a full tank and note your car's odometer reading at that point.
Do not record the gas you put into the tank at the beginning of this process. That was just to create a starting point. You will measure only the gas you put into the car after that point. Simply keep track of every gallon of gas you put into your car, regardless of whether you've filled the tank or not.
Each time you do fill the tank to the top again, make sure you record your odometer reading at that time. This becomes a new data point in your overall gas mileage measurement.
Each time you fill the tank, subtract the beginning odometer reading from the current reading to get the total miles driven. Then, total up all the gallons of fuel you've put into the car since the beginning point. Divide the total miles by the total gallons, and you've got your miles per gallon.
What about that second point? How can you make sure you have not forgotten to record your partitions or gallons of fuel purchased?
Well, the mileage takes care of itself. Your car's odometer faithfully records every mile you drive. As long as you wrote down the odometer reading at the beginning of the process, when you filled your gas tank, your car's odometer has a perfectly reliable record of how many miles you've driven since then.
But keeping a record of every gallon of gas you put in is really up to you. You need to form the habit of recording every gallon of gas you buy. It's not enough simply to keep your receipts. Sooner or later, you'll buy gas from a pump that is out of paper and you will not get a receipt.
How you record your gas purchases is up to you. You could carry an audio recorder and read the gallons off the pump whenever you buy gas. Or jot it down on a notepad or into your PDA. Send a text message to yourself. Whatever works for you, form a habit of recording every time you buy gas.
As you accumulate all this data, you'll almost certainly want a spreadsheet of some kind to do the calculations for you, and to keep a long-term record of your gas mileage. A simple spreadsheet with the appropriate formulas in it can give you several views of your gas mileage, such as your long-term gas mileage, your short-term gas mileage, and a moving average gas mileage over your last ten or so fill-ups . This will allow you to notice changes in your car's gas mileage so you can see when something is going wrong – such as when it's time for a tune-up – or so you can see that that new gizmo you installed is really improving your gas mileage .
Yes, measuring your car's gas mileage is really a simple thing, but you have to be sure to do it right. Start with a full tank at a known number of miles on your odometer, then be sure to keep track of every gallon of fuel you put into the car after that.