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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While the gauges can be useful, they typically report that your fuel economy is better than it actually is.
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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.
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
Carroll Lachnit, features editor, Edmunds
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.
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.
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.
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.
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.
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.
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."