Your Fuel Economy Gauge Is Fibbing

Edmunds.com

Your Fuel Economy Gauge Is Fibbing

While the gauges can be useful, they typically report that your fuel economy is better than it actually is.

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dr_science

Did you double check those numbers?

My calculations yield:

12000 miles / (25 miles per gallon) * (4 dollars per gallon) = $ 1920

Looks like there was an extra multiplication by 4 in your numbers, making the problem look 4 times worse.

brianalex

The mileage meter in my Mercury Grand Marquis is surprisingly accurate Except for one thing that should have been covered in the otherwise excellent article; The odometer in these cars also lies.
Every car I have ever driven over the years has had an odometer that reads high.
In my Merc it is 4% high even with brand new OEM tires. So although my fuel meter matches the odometer within 1% in most cases,it is still way off of the actual mileage covered.
Also it could be pointed out that for the average motorist to get the best results he/she would have to fill up at the same pump facing the same direction at the same temperature,and the underground tank temperature is the same, IE. no recent fuel delivery.
Bottom line; fuel meters are for finding the style of driving that yields the lowest number.-Brian

clachnit

Thanks, dr_science, for pointing out our math error. We've corrected the article with a different example--it's accurate this time.

Carroll Lachnit, features editor, Edmunds

spokybob

I know the 2011 Scion fuel usage is lying. I checked a few tanks full. It is almost 10 per cent. The best part about the gauge is the ability to determine that 87 octane gets about 7 percent better milage than 89 octane. (E-10)

vinvaz

I have seen that my 2010 Jetta TDI sedan's fuel economy guage is pretty accurate.. The error values are usually 1 - 2 mpg at worst. Are we talking about 5 mpg variations here?? My observations are mostly over a tank full, and involves a 50 - 50 mix of city and highway driving..

username_1

I drive a 2011 Hyundai Sonata in Queens, NYC. I have noticed disparities in my fuel economy gauge and my actual "calculator" economy at the pump. Though NYC's weather is a nightmare for MPG variation, I am very unhappy with this. EPA claims 22 city, 35 highway, 28 combined. I am averaging 20.1 MPG across the board over a 16 month period. This includes a few 300-500 mile road trips and a normal highway/city breakdown of about 60/40. My gauge consistently reads 15-25% over actual MPGs.

Buyer beware, this car gets insanely terrible fuel economy in the winter. I have seen entire tanks at 16 MPG! Worst part is the gauge reads, 20-22 MPG.

Sites like fuelly.com are our best bet. EPA estimates and MPG gauges are simply fraudulent. My 2000 Civic's EPA estimate was 27. Guess what, it got 27. New EPA standards need a serious re-work.

printerman1

fuel economy gauge is pure bull. Its a computer. if you drive city, you get crappy gas mileage. Cars, without computers, have not changed in years. Same bull, different pile. The computer drives the engine, not you!

carchatter1

Variances from the expected target is normal in everything, so this isn't really a big news flash. My 2 cars with trip computers both calculate the MPG to be accurate within 3 or 4 percent. A minivan and Mustang, both seem as accurate as can be expected. It would be difficult to predict perfectly what the real MPG was every time. Anyone expecting perfection is not living in the real world. As Brianalex noted, the odometers are more inaccurate than the MPG displayed. Now that would be a more newsworthy story, to show how few cars actually are going as fast as they say they are, and if the odometers are actually correct.

smkrmk

Your article may point to a bigger issue with car manufacturer's deceptive practices. I have leased three different model Nissans in a row (Murano, Altima & Roque). In each case, the computer calculated MPG is always higher than the actual MPG that I manually calculate.

I took my curiosity a step further when I noticed that my speedometer reads 2-3 MPH faster than what my GPS indicates and the same amount whenever there is a radar driven speed sign posted.

Knowing those two inaccuracies, I decided to check my odometer accuracy. When I was on the highway, using posted mile markers, after about every 3 miles my vehicle would read 3.1 miles higher.

My theory is this... If the manufacturers calculate higher mileage on the vehicle, it increases the frequency of maintenance intervals, it makes the warranty end sooner and it could put lessees into an overage situation sooner. If your 5.5% inaccuracy is true on the odometer also, a vehicle with a 36,000 mile warranty and/or lease would only be getting just over 34,000 actual miles before expiration or overages.

If this is intentional or and uncorrected issue with the manufacturers, it certainly is worthy of investigation and has the potential for financial damages to the consumers.

badhatharry

My 2013 Volvo C-30 computer showed an average of 32.0 MPG for the last 28 fill-ups. My actual calculations show 26.7 MPG. This is a skew of 19.9% - even higher than the "one such gauge" reported above.
So how do we fight this? Our local newspaper has a column where the auto reviewer reports fuel economy on tested vehicles, but he hasn't responded to my complaint. I discount these reports, as well as the claims of my friends on their new cars.

cdiman7

I disagree, the fuel gauge is accurate, for instantaneous MPG. It assumes that you will be driving the whole tank at the same road conditions,angle, air temperature, wind, precipitation level, humidity, sun, etc. Once I just filled up my 2006 Civic Hybrid, going downhill, summer day, no rain. It registered 150 MPG for a second,then went down to 50 MPG. So the gauge is right, just doesn't average the mpg so it'd be closer to mileage/gas used.

craig2008tl

My 2008 Acura TL grossly overestimates the fuel economy even according to it's own measurements. It's simple to check this, just fill the tank completely, reset the trip computer and then drive until the fuel light comes on. Divide distance by how many gallons you put in and compare that to what the trip computer claims is the mileage. Typically the trip computer will claim 20mpg when simple division shows more like 17mpg. Even allowing for rounding up that's at least 14% higher than reality, or put another way it's a shameless lie.

As for why they would do this, I can't believe the article feigns not to understand. It's because they're trying to make you think your car is more economical than it really is. Amazingly people actually prefer cars that have better fuel economy. Who knew? And all of that rubbish about fuel energy density and metering squirts at the fuel injectors or mileage variations due to tire sizes or inflation is just that, rubbish. They know how big the fuel tank is and so they know how much fuel is consumed when it goes from full to empty, and I'm using the car's own stated mileage, accurate or not.

Basically if you're going to even begin to talk about your car's fuel economy consider the mpg reported by the trip computer to be the pie in the sky fantasy number that they publish to entertain you. The real number is fuel used divided by distance travelled (and even that is better measured by an independent GPS since the odometer has also been found to overstate the actual distance), not the number the car self-reports.

blub

My Miata is right on the money after doing more than 20 fill ups along with my odo and speedo checked by GPS.

chris_matheny

I have a new 2017 Honda Ridgeline and the average MPG meter is consistently overstating by 1.8 mpg on every trip (5-10%). That tells me they calibrated to overstate on purpose, because the technology exists for these to be accurate. My 15 year old Dodge always calculated within +/- 0.5 accurate. I'm sure Honda could beat 0.5 accuracy if they wanted. Only reason I can think for manufacturers to purposely overstate is that they assume most owners don't know how to calculate actual usage (which is probably true) and are conned into thinking their car gets significantly better mpg than advertised on the sticker and therefore run around saying how great their (insert brand name here) car is on gas.

waynebengston

My Focus RS meter is consistently about 1 MPG high over the course of 19,000 miles.

bsmedesigner

I just purchased a used Subaru Impreza 2.0i Premium. During a road trip to Washington, DC from Charlottesville, VA, the fuel usage digital indicator on the dash was showing a whopping 46mpg! I was thinking to myself, "Wow, this is the best economy I've ever experienced in a gasoline fueled car!" The wishful thinking was squelched when I filled it up and did the mileage, manually. The ACTUAL mileage was 35mpg, which isn't bad, but I've never seen a digital readout on a vehicle THAT far off (about 30%)! My Toyota Avalon is dead nuts on read-out versus actual. Is there any way to improve the estimated mileage on the dash?

Alejandro

Hi. Usually, what the meter shows is an average of the mpg, more like a tendency. You can check this by simply resetting the meter and pressing and depressing the accelerator pedal a couple of times, then driving aggressively, and then nicely. you will see that the mpg changes very quickly. Its kind of difficult to understand what really happens because almost everybody in the U.S. drives a car with an automatic transmission. Let me be more specific:

As we all know, the transmission is what converts engine energy onto speed by transmitting the movement to the axles and then to the tires in order to start rolling. The transmission needs to be able to power the car from 0 MPH to whatever speed you decide to maintain. Lets use in this example 60 MPH as a cruise speed. If we open a transmission, what we will find among other things, is a few rotors that couples in between them, in order to move the car from one gear to the other one. Usually a car has 4-6 gears. In the first gear, with the same increase of engine RPM when we give it throttle, we can get to 10MPH +- from 0 MPH. the second one is for more speed at the same RPM, and so on until the last gear, which we set (at manual transmissions) or the car sets itself (at automatic transmissions) in order to have the most speed out of the less RPM. What consumes fuel, in deed is engine RPM. The greater the value, the most fuel the injector needs to spray into the combustion chamber faster enough so the engine can reach the Revolutions Per Minute (RPM) desired. Thats why transmissions uses its last gear. The greater the gear, the less energy is need to roll the vehicle at higher speeds.

Now that we know the basic function of a transmission, lets take a closer look to what happens at this situation: suppose you are traveling at 60 MPH and you need to speed up, or you will crash into a vehicle that is coming to you, so you press the throttle hard, and we can hear the engine at greater RPM maybe 2500-3500 as we gain speed. If we had a car with a manual transmission, and we are traveling at 5th gear, the way to do this is to manually downshift to 4th gear so we can get out of the trouble. Of course, 4th is stronger than 5th, by getting more speed using more engine RPM, and more engine RPM is more fuel. In an automatic transmission, happens the same (Unless in a CVT transmission that the gears system is different.). The computer of the car does the same, its just we didn't notice the last time it happened, or hold on, maybe we did. That second between the moment we press the pedal and the moment starts to gain speed again, thats the automatic downshift. So we have two scenarios, at high speed. One with low engine RPM (less fuel) at 5th gear, and the other with way more RPM (more fuel) at 4th gear. so if you travel at 60MPH and you drive aggressively, your car will downshift more, wasting more fuel to travel the same distance, that if you drive nice and smooth.

My point on this, is that fuel economy depends mostly on how we drive. is not exact. is like ETA on distances. it varies. On distances it depends on traffic, on fuel, depends on how heavy foot we have .

However, the cars computer have a way to compensate air-fuel mix when we have a problem in our car. Maybe the problem doesnt light the Engine Service Light, which by the way, there is a lot of problems that doesnt light that up, but one of the systoms that you car is putting more fuel into the mix, could be a poor Fuel Economy. So its better that you have your car checked when this is starting to happens.

And my last advice: try to find a technician that takes more time on diagnostics, than replacing parts. Remember, ignorant people can get to conclusions pretty fast and you will end yourself paying 600 bucks in parts, when the problem could be a simple vacuum leak.

PF_Flyer

I usually get right around the EPA estimates with my vehicles, so the dashboard numbers are sort of close, but not exact. It's an old habit, but I always calculate my mileage at each fillup, so I KNOW what mileage I'm getting. The mileage calculators in the dash are nice, but I really only use them as a gauge of whether there might be a problem brewing, same as my mileage calculations. If there's a sudden change in my mileage, something MIGHT be up. With my manual calculations, that usually turns out to be a short fillup the time before.
I think the most useful thing, for me, is the low level warning. Yes, it trips sometimes on a hill when the gas level isn't quite down to "better find a gas station", but I can account for that, and the warning IS helpful.

Autoputzer

I track my "gas pump" or real MPG on a spreadsheet. I reset the on-board computer (OBC) MPG every time I fill up. I've done this on four cars with OBC's for the last 18 years. I've calculated a correction factor (C) where:

Gas Pump MPG = OBC MPG x (1+C)

I calculate C for each tank of fuel and also C Since New, and C Since Adjustment. Here's the C Since New for my four cars:

2002 BMW M3 -5.0% (-0.05)
2007 Chevy Cobalt: +0.27% (+0027)
2014 BMW 535i: +1.8% (0.018)
2018 BMW X3 30i: -3.6% (-0.036)

The 2014 BMW 535i and the 2016 BMW X3 provide for adjusting the OBC MPG. I recently did the 2014 BMW's myself. The 2018 BMW is still under warranty, so I had the dealership do it. With both of these cars, C is drifting in the positive direction (negative and drifting toward zero, or positive and drifting higher). So, I "aimed low" when adjusting the OBC MPG.

After two tanks of fuel, the adjusted C for the 2018 BMW X3 is now -0.19% ((-0.0019). After one tank of fuel, the adjusted C for the 2014 BMW 535i is now -0.23% (-0.0023).

So, in the case of newer BMW's the answer to "How accurate is the OBC MPG?" is "as accurate as you want to make it be."