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Thursday, January 24, 2013

How to check your gas mileage: Step-by-step instructions to calculate miles per gallon (mpg)

Check you Gas Mileage and Save Money

Gas mileage is measured in mpg:  miles per gallon.  The calculation of miles per gallon is the miles driven divided by the gallons of fuel used to drive those miles.  This article provides step-by-step instructions with pictures to measure miles driven, measure gallons of gas consumed, and calculate gas mileage in miles per gallon.

Checking your miles per gallon is useful to figure out how much it costs to operate your car- this can be an important factor in deciding whether to get a different car.  Also, the mpg performance of your vehicle provides an indicator when maintenance may be needed- if your mpg dips this may indicate low tire pressure or a dirty air filter.  By tracking your mpg, you will be able to notice driving habits that lower your fuel efficiency.

MPG Monitor Mischief- Conspiracy Theory

Some newer cars automatically calculate and display gas mileage instantaneously as you drive on a fuel consumption display.  So if you have a built-in mpg monitor, you never need to check gas mileage yourself- right?  I would check it myself the old fashioned way, using my own measurements of miles driven and gallons of fuel consumed.  Why?  There has been at least one instance of a hybrid vehicle with inflated mpg ratings placed on the window sticker.  A high miles per gallon rating sells a lot of cars.  Carmakers have a huge incentive to display the highest mpg number on the indicator while you’re driving too.  If you test drive a car that displays a high mpg, you’re more likely to buy it.  After you buy it, if you think you’re getting great gas mileage, you’ll probably have higher satisfaction with the vehicle- and be more likely to recommend the vehicle and even buy another one.

Wouldn’t people notice if the mpg number  on the display is a little high all the time?  Unlikely.  How many people with an automatic mpg monitor ever manually check the mileage and compare?   Even if you have an automatic mpg monitor, check it yourself with these steps.

Step-by-step Instructions to Check your Gas Mileage

1. Reset trip counter at your next fill-up.  Fill the gas tank all the way full.
Most odometers have a trip counter.  This is an odometer that can be reset to zero by pushing a button.
Resetting the trip counter will start tracking miles driven on this tank of gas.

What if I don't have a trip counter?  You'll need to record the odometer reading now to use later.

Press button to reset trip counter to zero
Fill gas tank full and reset the trip counter
Image source: Dr. Penny Pincher

2. At your next fill-up, note how many gallons it takes to fill the tank all the way full.
You can get the gallons from reading the display on the pump, or from your receipt.

Fuel pump showing gallons of gas pumped
Record the gallons needed to fill the tank.  This is available on the pump display, or on your gas receipt.  The reading is 8.949 gallons.
Image source: Dr. Penny Pincher

3. Note how many miles are on your trip counter

Trip counter reads 330.7 miles
The trip counter with miles driven : 330.7 miles
Image source: Dr. Penny Pincher

Tip: write mileage from trip counter on the receipt so you can do the math later

Receipt with 330.7 miles written and 8.949 gallons of gas printed
The gas receipt shows gallons used to fill the tank.  You'll also need the miles driven from your trip counter.
Image source: Dr. Penny Pincher

What  if I don't have a trip counter?  Take the current odometer reading and subtract the reading you wrote down in step 1.  This is the number of miles driven on the current tank of gas.

4. Divide miles from trip counter by gallons from receipt or gas pump display.
Most people will want to use a calculator for this step.
For example, if your trip counter reads 330.7 miles and you used 8.949 gallons of gas, the calculation is:
330.7 miles   / 8.949 gallons  = 36.95 miles per gallon (mpg)

Calculator showing result of 36.95mpg
Tip: I keep a $1 calculator that my kids gave me in the car.  The calculator on a cell phone or computer will work as well.
Image source: Dr. Penny Pincher

Example calculation:
How to calculate miles per gallon using a calculator:

  1. enter 330.7
  2. press division key
  3. enter 8.949
  4. press equals key

The answer shows your gas mileage in miles per gallon, 36.95 in this example.

5.  Important- reset trip counter after you note the miles to start tracking miles for the next tank

You will need to reset the trip counter while you're still at the gas station to record the miles driven on you next tank.

Recommended reading:
How to Buy a Used Pickup Truck
How Much Would I Save on Gas with a More Fuel Efficient Car?
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