How to Calculate your Vehicle’s MPG

Last week, I showed you the progress I am making on tracking the fuel efficiency of my 2015 Corolla! So far I am getting the results I wanted, with efficiency at or above the highway rating of 37mpg for my vehicle.

So… How do I calculate this?

Most modern vehicles already calculate MPG as well as other vehicle stats on the media display or the dashboard. But this virtual MPG isn’t always accurate. I am going to show you how to calculate your vehicle’s MPG yourself, as well as the cost per mile driven. Even if you know absolutely nothing about cars, you will be able to do this after you read through this guide!

Before I get into the numbers, I want to lay out all the terms I will be using:

  • MPG (miles per gallon) = Fuel Efficiency. This is how many miles your vehicle driving for each gallon of gas. In USA, the EPA does testing on each vehicle in different scenarios to give each car an estimated MPG rating. EPA ratings are only that… testing done by an engineer or technician on a vehicle. So this means that any particular driver could be able to achieve the EPA rating, but it also means you could achieve a better or worse MPG rating. The key here is to remember that EPA testing is done under xyz scenarios to determine the MPG rating. You, the smart and efficient driver, have control over your driving.
  • CPG (cost per gallon) = This is the price per gallon you pay for gas at the pump.
  • CPM (cost per mile) = This is the price each mile of driving is costing you. This is useful for creating monthly budgets or when deciding to buy a new car. I’ll explain more on that later. CPM won’t be given to you anywhere, so we have to do a little math here. To calculate CPM, just divide CPG by MPG. It’s that simple.

With that out of the way, how to calculate MPG? I’ll show you with an example:

Next time you fill up (all the way) at the gas pump, take note of the odometer and the CPG.

Gas Refuel #1:   10,000 miles  CPG = $3.00

When you get low on gas, refuel all the way to full, again take note of the odometer, the CPG, and the total gas you just dumped in the car. The pump screen will show you the total gallons.

Gas Refuel #1:   10,000 miles  CPG = $3.00

Gas Refuel #2:   10,300 miles  CPG = $3.00  Fuel: 10 gallons

(I am making these numbers up for the example, not a real car)

With these two data points you are ready to calculate MPG. Between refuel #1 and #2, our car drove 300 miles (10300 – 10000) and used 10 gal.

MPG = 300/10 = 30

Easy right?

Gas Refuel #3:   10,650 miles  CPG = $3.00  Fuel: 9.8 gallons

10650 – 10300 = 350             MPG = 350/9.8 = 35

Each refuel, you can calculate the MPG for the distance driven since the previous refueling until the current refueling. For our 2 complete trips, the average MPG is 32.85. Trip #3 MPG can’t be calculated until Refuel #4, for which reason it is not included in the average MPG.

As for CPM, earlier I said it is calculated by dividing CPG by MPG.

Trip #1 CPM = 3/30 = $0.10

Trip #2 CPM = 3/35.7 = $0.084

Since both completed trips had the same CPG (cost per gallon), the average CPG is $3.00.

Average CPM = 3/32.85 = $0.091

If CPM = $0.10 per mile, and I drive 10,000 miles a year, I spend $1,000 on gas a year.

If CPM = $0.084 per mile, and I drive 10,000 miles a year, I spend $840 on gas a year.

This is why CPM is helpful. Notice the better your MPG, the less you spend on fuel. Gas prices obvious affect CPM too, but you can only control your MPG, not the gas price.

Let’s say the car we used in this example, monstermobile, is averaging 25mpg after a few months of refuels (more than two data points for a much more accurate average result) and has a CPM of $0.12 (gas price still at $3.00). The EPA suggested MPG on monstermobile, is 30mpg. This tells us either we are driving inefficiently, or there is something wrong with the car!

Since Monster drives 50,000 miles a year, that’s $6,000 a year in gas! Or $500 for doing your budget. Thinking on monstermobile’s demise, I go car shopping and notice all the new cars have a sticker with the EPA suggested MPG, and choose one with a suggested 33mpg. This car would have a potential CPM of 3/33 = $0.091, and reducing driving to a more reasonable 15,000 miles a year, this new car would spend $1,365 a year in gas! Much better than the $6k.

I’m not saying to go buy a new car, but to demonstrate how knowing the MPG and CPM can be useful for every driver.

Keep driving smartly!

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