Gas Mileage Discrepancy

Try taking the average over multiple tanks (add up all the gallons and all the miles) and you should get a more accurate measurement.

There are 2 things can can be measured incorrectly here, and both are done by the car itself.

Firstly, the car cannot exactly measure the amount of gasoline in the tank. it uses a (very imprecise) float-type apparatus to measure the amount of gasoline in the tank (and how much is used).

Secondly, the MPG measurement is based on the *estimated* amount of gas used (taken from that imprecise float-gauge in the gas tank), the rpm's of the transmission, what gear you are in, etc.

The best way to get a true measure of your MPG is to take data from 2 fill ups and do the math.
1) Fill your tank with gas (even the hippie-friendly California gas nozzles will do)
2) Record your odometer reading.
3) Drive your car until you need more gas (doesn't matter how far, could be just a few miles, or maybe use just a few gallons, or maybe you could drive it till its almost empty)>
4) On your NEXT fill up, record the odometer reading again. Subtract your last odometer reading, and you will have the exact number of miles you've driven.
5) On this same fill up, record the number of gallons added to your car (read from the pump)
6) Divide the number of miles you've driven by the number of gallons you pumped. Thats your mpg.

If you use the same pump on both fillups (eg, one close to your home), you should have a pretty accurate MPG. If you do this everytime you fill up, you can track your MPG over time.

I realize that pumps don't always cut off at the same time for every fill up, but they do measure pretty accurately the amount of gas that they dump into your tank. Especially if you use the same nozzle 2 times in a row, you're almost guaranteed to get an exact # because more than likely they'll cut off at the same relative level.

In theory the computer knows _exactly_ how much fuel is being consumed, it can be calculated from the fuel pressure and the amount of time each injector is opened. However, in the real world this is not an exact science, fuel injectors leak or lag, pressure sensors can be inaccurate, etc. The "miles till empty" is more of a black art since it is relying on the fuel gauge sender to estimate the amount left in the tank.

Another car-based error comes from the way the distance-travelled is measured. The input for the odometer, speedometer and mileage meter are all based on final drive shaft rotations. Different tire models, wear, even pressure will produce more or less miles travelled per drive shaft rotation. You can see how far you're off by comparing your odometer readings to the distance announced by road signs (use long distances and/or averages).

But the flow meter (if one is used) will produce more error. The fuel flow meters used in aircraft have to be very accurate (temperature and pressure compensated) - pilots cannot pull over and walk to the nearest gas station. These flow meters tend to be spendy (several $100), so I doubt they use those in cars.

To get a nearly perfectly accurate reading try this... (it's a bit extreme) When your car is almost out of gas, drive around the block near the gas station you regularly use until your car completely runs out of gas... then putter on over to the gas station using what's left in the batteries to electric motorize yourself to the gas pump since you ran your gas tank completely empty. Then fill up with as much gas as you like (the more you put in the more accurate your reading will be) so fill it up. Then drive until you are almost out of gas again and go to the *same* gas station - not because of the vapor cut off people mentioned, but because it will be the same elevation as where you started (another important factor for true fuel economy testing since if you end your trip 200 feet lower than where you started, you are essentially cheating.) So when you get back to that gas station - drive around the block again until your engine dies again. Assuming your battery is at the same level as it was when the engine ran bone dry the first time before you filled up to test your mileage, you will get a perfect reading. Just check how many miles you drove and divide that by how many gallons of gas you put in. Hope that made sense!

Speaking of running bone dry - is this bad for the engine since it sputters before it completely dies? does that cause extra wear on the cylinders? email me - i love to get random emails on anything having to do with cars, especially on the topic of MPG! And thanks to the previous posters since I didn't know all those fine details. My '02 Prius also regularly posts 2 to 5mpg more than the odometer bears out. :( I haven't really tested this enough to know for sure tho, but the computer seems to be off. Maybe I will try my bone dry method - someone email me with concerns or advice or comments or questions! Thanks!

I have 2 HCH's (both manual), one for nearly 3 years. The computer-indicated mileage is routinely high, in some cases by as much as 10%. My 2005 has averaged an actual 45.53 over 8,223 miles; the car computer says 46.5, so that's about a 2% overstatement on average.
Incidentally, I also had an Audi A4 with mileage computer; it also routinely indicated better mileage than the car was actually getting. Can't imagine any manufacturer calibrating their system to indicate worse mileage than you're actually getting!

The bladder explanation is the one that was given to us by our Toyota dealer.

I happen to have filled my gas tank today in my 2003 HCH - it took 11.707 gallons. The tripmeter showed 482.2 miles since the last fill-up. The calculated MPG is 41.2 - the tripmeter said MPG was 43.1. 4.5% is a pretty significant discrepancy.

I own a 2003 HCH (manual) and have always noted an mpg discrepency. I think Steve's explanation makes sense with the fuel nozzle cut-off, and would explain the variability I see at different service stations.

In terms of the on-board mpg calculation, I am almost certain it comes from the fuel injection system, not the tank float device. I don't know if there is an actual fuel injection flow meter (probably not) or if it simply uses the 'requested fuel injection amount' from the controller. Regardless, I'm confident that it is fairly accurate. Modern engine controls need very accurate fuel injection algorithms to meet the stringent fuel exhaust requirements (especially SULEV levels in CA).

Regarding the last commet on engine idling, it actually is accounted for in your on-board mpg calculation. if you ever idle for extended periods of time, you will see the dash mpg number drop while you are not moving (fuel consumption going up while distance not changing).

My 4 cents, good discussion.
Joshua
(Davis CA)

Most cars use the odometer and throttle position to determine mileage. The computer calculates using these two inputs. The gas pump is much more accurate than anything on a car. All pumps will fill to the top. You need to calculate you mileage the same way everyone else does when they want accuracy. Divide the number of miles by the number of gallons and adjust for odometer error if necessary. The mileage computer is there to amuse us humans who believe that anything digital must be right.

I have owned a 2005 Hybrid Civic since Feb. of this year, 2005. I continually see an increase in my gas mileage. I commute almost 100 miles a day and think that my purchase is a rather smart investment. I filled my tank yesterday and I am now at 65 mpg. I think that this is GREAT!!! I have found that driving with my car on cruise at 45mph really is the best way to drive. I just wish that everyone drove a hybrid.

The dashboard gauge on my 2003 HCH Manual quite accurately calculated the mileage for the first year and 30k miles or so ('accurately' as calculated from the gas pump). At some point the dashboard gauge started being off by 5 to 10%. In warm weather, the gauge will typically read 51-53 MPG over a tank-full, while the MPG calculated from the gas pump will be ca. 47 MPG. I asked the Honda service advisor what the gauge measured that could be off and he said something vague about these things not being very accurate. Yes, I understand, but why was it accurate before?! Except for this I love my car. I have 78K on it and want to know why anyone would by something else.

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I have an 2007 Honda Civic Hybrid, and I find that its computed mpg is close to actual. It understates the actual, so you get a pleasant surprise.

I had a Toyota that was always computing mpg that was 15% higher than actual. It was that way since day 1, with of course stock rims and tires. Actually, stock everything.

After the low fuel light came on I filled up the tank all the way up the filler neck and pumped in 15.4 gallons. Is this possbile for the 13.4 gallon tank on my '05 HCH?

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