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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.
Or, in a recent case with my last car, if a fuel line sprung a leak due to rust/corrosion. I noticed immediately when my MPG went from 28 to 21
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!
Its only an estimate, I expected no more. How accurate was your 69 Chevrolet????
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.
can't imaging anyone in the "gas pump industry" would advise you to completely fill to the top of your tank. Not only a negligible difference in overall calculations it would lead to an environmental disaster if everyone were to do this. I have been in the habit of calculation my MPG since my first car I owned (1972 Ford Galaxie 500) had a faulty gas gauge. I have had dozens of vehicles since then and still continue to this day. I have found most have been within 5% accurate some more accurate than others. My last BMW would give me ratings that were over around 10% it also had a speedometer that was 2 MPH fast (not sure if this is related) many forums also revealed BMW's to have optimistic speedometers but fairly accurate MPG gauges. On the other side my wife has a 2021 Mazda CX-5 that consistently gets better MPG than the meter shows. Back to the reason I replied to this post, DO NOT TOP OFF your tanks please. This not only contributes to ground pollution buy vapor release into the environment. It may seem like a small thing but when practiced millions of times per day this is a very bad idea!! There is a reason they have auto-stop AND vapor collection on fuel pumps.
Thiis is AWESOME!!! I have not checked them all but I drove a Ram 1500 for 200k and now am on a 2500 diesel. Both are consistent at 10-12% below what the dash says. My last trip I just filled up 5 times on a 2400 mile trip and the dash gave me 25.1 but the actual by way of a calculator was 21. Over my gas and my diesel watching closely for a very long time I believe they are calibrated at milage plus 20%
“We explain why your digital miles per gallon display in your vehicle is not accurate.” Do you? The article demonstrates that it is common, but it doesn’t explain why. But it got me to click, so job well done I guess.
I have to agree totally. I'm going with the fact that the manufacturer's are LYING!!!!!!! My new Ford Ranger is 7 months old and I have been tracking the MPG manually each fill-up and the sensor is stating about 2 MPG over the true MPG.
I've owned a hybrid Ford Escape Hybrid and I am shopping for a plug in hybrid. I've recently spent about 8 hours researching and comparing PHEV vehicles. If you go on to the EPA website, it will tell you that mpg (e) are calculated for one driver, no load on a flat road (city/hwy) so for example, if you commonly drive 2 people and carry and extra 40 lbs of junk in your trunk, traveling up and down hills, in a very cold or very hot environment with the A/C on and improperly inflated tires, your mpg is going to be lower than stated. This has also been my experience with the fuel economy of my vehicle under any of the above listed conditions. Some people drive with multiple conditions just listed and expect the car to perform the same as controlled situations.
Any way you calculate fuel millage isn't going to be absolute. It's a rough estimate to better keep track of any changes indicating a issue. I guess one could argue that doing this yourself would be as accurate as the vehicles calculation. One wonders if this also affects how many mile you have left before empty? In my experience the vehicles MPG calculator has always given a optimistic MPG compared to my own figure.
2012 Honda CR-V computer, in city driving, was always correct, according to manual calculation. Highway it always show 1 or even more mpg less. With higher speed computer was more pessimistic. But driving 55 mph mpg was correct. 2010 Prius always optimistic by 6-7%. I noticed, most incorrect, when driving in EV mode. Computer mpg going up much more than on scan gauge.
I have a new Toyota Camry 2022 and the display shows 27.5 miles per gallon but I just filled up and figured out I am getting 30.2 miles per gallon.
Posted average fuel economies are great when choosing a car, but I think it only matters when you're choosing between 6 cylinder turbo and a 4 cylinder 1.5 L. That's where you will see significant savings. No matter how accurate the average fuel economy might be, I think the most important thing is to do what you can to keep it as low as possible on each trip. Drive slower, accelerate more smoothly, combine your errands into as few trips as possible, drive less. This is where you'll find significant savings rather than nitpicking over average MPG accuracy here and there. My RAV4 hybrid can have amazing fuel economy if I just drive better. When I'm late for an appointment and time is of the essence, my fuel economy sucks. In summary.... pick a car with smaller engine (hybrid even better, PHEV perhaps best of all) and drive in a more efficient manner, or drive less. From a fuel nerd perspective, what I'd really like to see on my RAV4 hybrid is a report on the EXACT amount of fuel I burned per trip, rather than averages. Does anyone know if there is a plug-in diagnostic tool that could report on actual fuel consumed?
Not mentioned above yet is fuel pump calibration. Certainly no vendor would actually sell less fuel than the pump display showed, would they? Huh?
In regards to the pump gallon display accuracy, here in MA, our inspector of weights and measures certifies the accuracy by law. Do you know something different? In regards to basic accuracy, I have kept track if the variation between indicated MPG and calculated MPG over 10,000 miles in my Mazda CX-5. On the average, the indicated MPG is about 6% higher than the calculated (miles / gal) MPG. Again, the question is WHY? Technical or marketing? Who Knows?