Should You Trust Your Car’s MPG Display?

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Our testing reveals an interesting trend. Cars have very optimistic MPG displays.

If you’re a fuel economy nerd like us, you may like to use your car’s “Average” MPG display to see how well you are doing. All new cars now have a way to zero out the MPG average. On some, it can be automatically reset when one fills up the car. On all the rest, one simply zeros it out using the trip computer to restart the average. Over the first few miles, it is meaningless, but fun to watch. Pull out on a long stretch of slow open road and the MPG average is amazingly high. Then stop a light or hit some stop and go traffic and see it drop down to an amazingly low number.  Over the full course of a tank of fuel the MPG average displayed should be pretty accurate though. What we have discovered is that on a wide variety of makes and models, the MPG Average is almost always very optimistic, and in some cases arguably misleading.

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When we review a car, we use the same method to calculate fuel economy every time:

Fill the tank until the nozzle clicks.

Zero out the trip odometer.

Drive a few hundred miles and refill the tank.

Calculating actual miles per gallon utilized is as simple and dividing the miles by the gallons consumed.

Some publications go much further and try to report a cars “actual” MPG, rather than the observed MPG we report. For many years Motor Trend has been tracking what it calls “Real MPG” in order to eliminate all sorts of theoretical things that might impact the simple method we use, such as the expansion of the fuel in the tank due to temperature changes and fuel energy density variations from station to station. The publication goes so far as to hang a big apparatus off the back of the vehicle, which of course impacts the car’s aerodynamics and coefficient of drag and introduces its own errors. We have never understood why anyone would go so far to try to make a simple thing more complicated than necessary, but we are starting to.

When we tested a Volkswagen Golf TDI (diesel) in 2014, we kept careful track of our mileage. VW TDI owners loved to report their amazing mileage numbers so we tried hard to replicate them. We ran the Golf TDI on the highway, using cruise control as much as possible, obeyed the speed limit, and left the AC off and the windows up. The car told us the MPG was 56 MPG. However, by our calculations dividing miles by gallons, we came up with 50.2 MPG, a 10% lower MPG. From that point forward we took careful note of the MPG display in high-mileage vehicles we tested.

Our most recent diesel vehicle was the 2017 Land Rover Discovery HSE Luxury Td6. The Land Rover does return very impressive MPG for a three-row tow monster. One tester covered 244 miles using 9.6 gallons of fuel, which equals 25.4 MPG. The dashboard reading was 28.1 MPG, or about 10% higher than observed. A diesel Jaguar XE 20D Prestige we tested last year covered 337 miles on 7.7 gallons of fuel. Thus, our mileage was 43.8 MPG. The Jaguar’s display read 46.8 MPG, a 6.5% difference.

Diesels are not the only types of cars with optimistic MPG displays. We tested the new 2017 Hyundai Ioniq Limited this year and were hoping to see it return the best-in-America fuel economy that Hyundai is now advertising. We were not able to reproduce the EPA’s estimated 55 MPG. Over 221.9 miles we consumed 4.6 gallons of fuel. So, our observed mileage was 48.2 MPG. The Ioniq’s display told us we had gotten 52.7 MPG, an 8.5% difference.

The most recent fuel-sipper we tested is the 2017 Acura MDX Sport Hybrid AWD Advance. We covered 307 miles with 11.14 gallons of gas. Our observed mileage in the MDX was 27.6 MPG (higher than the EPA estimate), but the MDX told us we had earned 29.7 MPG over that span, about a 7% difference.

The results: every vehicle we’ve tested has returned a significantly lower mile-per-gallon figure than the one displayed on the dash. If this was simply some kind of rounding error, you’d think that there would be a few cars that provide better fuel economy than the car recorded, but not one of the cars we have ever evaluated has ever delivered higher fuel economy than the one that was displayed.

Your mileage may indeed vary.