How Does My Car Calculate Fuel Economy?

Modern cars have dash consoles full of useful (and sometimes not-so-useful) information…like your MPG averages. How does your car know how many miles per gallon you’ve used? Let’s take a look…

How You Would Calculate MPGs

Before the days of computer-controlled vehicles, you would have to manually calculate miles per gallon…and the formula is in the name:

The number of miles you traveled <divided by> the number of gallons used

This formula is most easily calculated based on the number of miles between fill-ups. So, to find your average MPG:

  1. Take down the odometer reading at fill-up #1

Example: 47,882

  1. Take down the odometer reading at the next fill-up

Example: 48,180

  1. Record the amount of gas you replaced in your tank during the second fill-up

Example: 11.963 gallons

  1. Subtract the first figure (first odometer reading) from the second to get the number of miles traveled.

48,180-47,882=298 miles

  1. Divide the number of miles by the number of gallons of gas you replaced during the last fill up.

The answer is your MPGs: 298 miles / 11.963 gallons = 24.91 mpg…not too shabby but there’s room for improvement.

How Your Car Calculates MPGs

You’ll notice you do not have to enter any numbers for your car to spit out MPG averages. That’s because it’s already keeping track.

Your vehicle’s odometer records how far you’ve travelled. Your vehicle’s computer also knows how much fuel you use. It’s a little bit complicated to explain, but essentially because your vehicle’s fuel injection system is computer controlled, your car can calculate total fuel consumption based on the total “on” time of your fuel injectors.

If you watch your car-calculated MPGs, you’ll notice they are extremely low to start. The longer you drive, the more they tend to level out to averages that look more like your manual calculations.

Which Is Right?

If you compare your MPG calculations to the average MPGs your car calculates (not the instant MPGs if you have two separate readings), you’ll likely find they are comparable, so both are more or less right.

However, if you notice that either calculation is way off, then there are a few possibilities:

  1. Double check your math. It’s simple algebra, but it’s probably been a while since you’ve had to do anything like an MPG calculation, so just re-run the numbers with a calculator…or two.
  2. If your calculations are significantly higher or lower than your vehicle’s, something in your car’s computer may need to be recalibrated. Likely, it’s an issue that requires advanced diagnostics. So, bring your car to Quanz Auto Body.

If both your manual calculations and your car’s calculations show MPGs that are lower than expected for your vehicle’s make/model/year, then something else may be affecting your fuel economy—like bad spark plugs, clogged fuel injectors, etc. Contact Quanz Auto Body to schedule a mechanical inspection. We’ll find whatever’s killing your MPGs and give you a free estimate for maintenance and repairs.