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The Digital MPG Fuel Economy Display In Your Vehicle Is Lying To You

We explain why your digital miles per gallon display in your vehicle is not accurate.
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Fuel economy displays in our vehicles offer a neat bit of information. Who doesn’t want to know how many miles per gallon they are getting? The problem is, these in-vehicle average MPG displays are almost always inaccurate. In almost all cases, we have found through testing that they are wildly optimistic. In other words, they are lying to you.

We’ve compared the average MPG displays in our test vehicles to the actual miles traveled per gallons consumed for years. Over hundreds of vehicles tested, we have only seen the display be accurate a handful of times. Most of the time, the MPG display you see in your dash display is telling you your mileage is about 10 to 15% better than it actually is.

In a 2016 Subaru Forester 2.5i Premium that I own, the display is almost always about 1 MPG, or about 3% high compared to what the vehicle actually achieves for MPG. It likes to tell me the vehicle has a 30 MPG average over a tank, but when I compare that using miles traveled to gallons consumed, it is almost always about 29 MPG. As MGP displays go, this is one of the more accurate ones we have seen.

In a test of an all-new 2021 model year crossover, we zeroed the average and the trip odometer and set out on our test. The MPG display had us preparing to write a story saying this vehicle is as efficient as a hybrid. However, when the MPG display went above 40 MPG, we started to wonder if it could possibly be correct. Over the full test, it settled in at 35.8 MPG. We traveled 241.6 miles using 7.43 gallons of fuel, so our true mileage was 32.5 MPG. That is a 10% difference, and typical in its optimism based on our testing.

One of the highest MPG scores we ever observed was in a test of diesel Volkswagen Golf. The in-dash display told us the average was 56 MPG. However, by our calculations dividing miles by gallons, we came up with 50.2 MPG, a 10% lower MPG. Still a high number, but why does the car have to exaggerate?

Interestingly, one of the rare examples of a vehicle displaying a lower than actual fuel economy was in the all-new Honda CR-V Hybrid. We recorded a 39.09375 MPG number during our time with the CR-V Hybrid. Let's just round that to 39.1 MPG. However, the dash displayed 36.3 MPG.

In our testing, we, of course, zero the MPG average using the infotainment settings, and we fill the tank to the first click and zero the trip odometer. Next, we drive for the majority of the tank. We like to see at least 250 miles of distance before we check mileage. We then re-fill to the first click and we record the gallons consumed. Try this in your own vehicle and tell us in the comments below how close to actual your in-dash MPG display is.

John Goreham is a long-time New England Motor Press Association member and recovering engineer. Following his engineering program, John also completed a marketing program at Northeastern University and worked with automotive component manufacturers. In addition to Torque News, John's work has appeared in print in dozens of American newspapers and he provides reviews to many vehicle shopping sites. You can follow John on Twitter, and view his credentials at Linkedin


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Comments

I have lobbied fuelly.com to add an "indicated MPG" field to their fuel tracker for a very long time. I think it would be a great data point to see which manufacturers are inflating (either intentionally or not) their in-vehicle MPG displays versus what the car is actually getting. I know it's all just math based on injector timing, RPM, speed, and fuel level, but the more advanced the electronics get, the more accurate the MPG should be (like maybe incorporating GPS location in the calculation). One very important reason to track your actual mileage is that a decrease in mileage is almost always the first indication something is wrong with your vehicle. It could be a slow leak in a tire, oil getting in a cylinder head, or (like just happened to my 1 year old car) a battery going bad. My in-car mileage didn't change from ~22, but the last 3 tanks had actual mileage of 19, 20, then 16 -- then the battery died and it wouldn't keep more than 4 volts in it.
The only problem with calculating the actual fuel economy using gallons pumped and the trip odometer number is how accurate those readings are. Do you have any info on that?
I don't have any issues with calculating actual fuel economy, but the info on assurance you desire is: - On the weights and measures sticker on the pump at your gas station - On your tire manufacturers revs-per-mile specification for the model and size you bought - Your comparison against the OEM tire that came with your car - The comparison you did when driving using the mile markers to ensure your odometer was correctly calibrated from the factory.
Very good point!
There is only one way to be accurate when determining fuel mileage. And that is to fill the tank until it can no longer hold anymore fuel before you start driving and when you refill the tank. According to people in the gas pump industry, you will not get the same amount of fuel in your tank by stopping at the first click as you say. Depending on conditions with the fuel tanks and weather temperatures the gas pump, stopping at the first click is going to differ from one station to another. Filling to the top each time and dividing the number of miles driven by the amount of fuel put into tank is the only truly accurate method to determine gas mileage. My 2020 Lexus 450H is without a doubt the most accurate when it comes to stated mpg on fuel gauge vs actual mileage. If my fuel gauge shows 28.? Mpg, The true mileage will actually be 28.? mpg. I have owned over 40 vehicles and I have checked every tank for mileage the last 20 years.