How accurate is the Ave mpg?

[ncsu]

Has anyone been comparing the cars computer calculated mpg to manual calculated mpg?

I have put gas in my car 2 times. I have found that the car's mpg is on the optomistic side.

Tank 1 - 4.0% highTank 2 - 6.5% high

Has anyone else seen this same error?

[Pescakl1]

The system is as accurate as the components which compose it... meaning, not accurate at all.

If you go in any car forum, you will see it is the most common critic.

The system is not here to tell you how many centiliters of fuel is left in your car, but it is just an INDICATION that you can do your planned trip without fearing to be stranded on the side of the road because you are out of fuel.

It is all depend how people fill up their tanks, what pump they use, how hot or cold it is, etc...

Even for planes and race cars, they are not able to tell accurately the amount of fuel they have on board, so don't expect that from a sub 30.000$ car.

You now know that your computer is about 5% optimistic, that is the most information you can have from this system. Use this information if you want to go to the limit of your fuel tank.

[Elton Noway]

Has anyone been comparing the cars computer calculated mpg to manual calculated mpg?

I have put gas in my car 2 times. I have found that the car's mpg is on the optomistic side.

Tank 1 - 4.0% highTank 2 - 6.5% high

Has anyone else seen this same error?

When I first got my Rogue, after the first couple fill ups, I also found the computed MPG was higher than what I calculated. However by tanks five and six the computed versus calculated have been relatively close to each other... within 1 to 2 MPG of each other, with the computed always being slightly higher.

NOTE: Be aware if you do not reset the computer... the "computed" MPG will be a running total. In other words ... if you take a LONG trip on one tank of gas (yielding better MPG than city driving) but your very next tank full is used driving around the city with a lot of stop and go)… the computed MPG for the last tank will be "skewed" because it is factoring in the MPG of the previous tank full. To make sure I get an apples to apples comparison (computed versus calculated) I always reset the MPG computer reading back to “0” at each fill up.

NOTE: I also make sure I always fill the tank to the “same” fluid level every fill. Good or bad I fill it up to the top of the filler neck… less the depth of the gas nozzle. (I’ve read concerns about leaving room for expansion etc… but thru the heat of the summer have been filling it this way with absolutely no problems.) Anyway… filling it to the same level each fill up gives me a more accurate indication of MPG performance from one tank to the next. For example: If on one tank I filled it to the rim… but on the next tank I stopped filling it when the pump shut off… then my calculated MPG would not be as good on the second tank because on the previous tank I squeezed in an extra gallon or two.

[koolyce]

I keep a log of all my fill ups and now, at 5500km, my computer is about 0.4L/100km lower than my manual calculation.

[ncsu]

Even for planes and race cars, they are not able to tell accurately the amount of fuel they have on board, so don't expect that from a sub 30.000$ car.

.

Actually most race cars can tell exactly how much fuel is in the car - with the exception of NASCAR since they have outlawed any technology developed after 1976

I would expect that the mpg (not dte) would be tied into the ECU since it measures the amount of fuel burned, but maybe my expectations are too high.

[koolyce]

Actually most race cars can tell exactly how much fuel is in the car - with the exception of NASCAR since they have outlawed any technology developed after 1976

I would expect that the mpg (not dte) would be tied into the ECU since it measures the amount of fuel burned, but maybe my expectations are too high.

Well, in F1 the only thing they use to calculate how many L left the car have is by using the current consumption with how many L they put in the tank. They don't know how many L they have left in the tank. Because if they know, why we see some time guy runnint out of gaz during a course?

[philipa_240sx]

I would expect that the mpg (not dte) would be tied into the ECU since it measures the amount of fuel burned, but maybe my expectations are too high.

No, your expectations are correct. The ECU communicates with the trip computer and provides average fuel consumption rate. The trip computer then uses fuel consumption rate, vehicle speed (provided by the ABS computer) and the fuel level sensor to calculate DTE, avg. fuel consumption, etc.

There are inaccuracies in all of the information provided to the trip computer. We can eliminate the odometer/speedometer as one source since both the trip computer and our hand calculations use this source. There are a few others:

- Repeatability of filling up to the exact same tank level. Even a half gallon difference can skew the reading by several precent. Hence why you should always use the same pump and filling method to reduce this error.

- ECU fuel consumption is an estimate and is averaged over time. Part of the issue here is the ECU doesn't actually 'measure' the amount of fuel delivered. It only adjusts it based on airflow (mass airflow sensor) and air/fuel ratio. Engine manifold vacuum also plays a big role in how much fuel is delivered to the engine.

Important side note:

Older fuel injection systems used a vacuum referenced fuel pressure regulator. As engine vacuum increased (during deceleration for example), the fuel pressure drops so approximately the same quantity of fuel is delivered with the same fuel injector pulse time. This can make the fuel consumption calculation more accurate as the ECU has a good idea how much fuel it has delivered.

Modern emission controls have changed this. Almost all cars made in the past few years use a constant pressure fuel system. As engine manifold vacuum increases, more fuel is delivered. The ECU doesn't know how much more fuel is delivered nor does it care, it simply reads the sensors and adjusts the amount to ensure the air/fuel ratio is correct. The fuel consumption rate can only be estimated now. It's never going to be 100% accurate. Maybe a 5% variance is as close as it can get.

[Pescakl1]

Well, in F1 the only thing they use to calculate how many L left the car have is by using the current consumption with how many L they put in the tank. They don't know how many L they have left in the tank. Because if they know, why we see some time guy runnint out of gaz during a course?

Exactly my point: For aircrafts and race cars, they calculate the amount of fuel in the tank(s) by subtracting how much they put in by the amount they are using.They never know for sure how much they have in their tank(s). Depending of the accuracy of the fuel meter (drifting of the sensor with time for example), they could run into trouble.That is why, they regularly empty the tank(s) to be sure of the exact amount of fuel they have when they refuel.

So, don't expect too much from your car.

[Elton Noway]

I would expect that the mpg (not dte) would be tied into the ECU since it measures the amount of fuel burned, but maybe my expectations are too high.

Well, I'd care more about how DTE (distance to empty) was calculated if it were good for anything, I could care less if I have 330 miles DTE when my tank is full, besides gas stations are not that far apart. I might hit a stretch of road where they are 75 miles apart, but I can plan accordingly. Personally I'd rather rely on DTE when my fuel tank is getting low. Problem is (in my Rogue anyway) its all but worthless at that point.

Whenever my low fuel light comes on... the mpg reading goes away and my DTE mileage starts flashing. It always does the same thing. The low fuel warning light comes on and then the DTE displays between 40 and 44 miles. When DTE gets down to about 30 miles... the DTE display goes blank and changes to a couple little dashes. So... I have no longer have a calculated indication of how far I could potentially travel. Good thing I have a trip meter. When the DTE goes to dashes I simply monitor the remaining calculated distance with the trip meter.

The ECU communicates with the trip computer and provides average fuel consumption rate. The trip computer then uses fuel consumption rate, vehicle speed (provided by the ABS computer) and the fuel level sensor to calculate DTE, avg. fuel consumption, etc.

- ECU fuel consumption is an estimate and is averaged over time. Part of the issue here is the ECU doesn't actually 'measure' the amount of fuel delivered. It only adjusts it based on airflow (mass airflow sensor) and air/fuel ratio. Engine manifold vacuum also plays a big role in how much fuel is delivered to the engine.

The ECU doesn't know how much more fuel is delivered nor does it care, it simply reads the sensors and adjusts the amount to ensure the air/fuel ratio is correct. The fuel consumption rate can only be estimated now. It's never going to be 100% accurate. Maybe a 5% variance is as close as it can get.

philipa_240sx ... Your ability to succinctly delivery an accurate an articulated answer never ceases to amaze me. Nice write up!