Indicated MPG vs. Real MPG

[BlueReflex]

I thought I'd share my experience to see if anyone else has noticed this. My indicated MPG was 25.2 this tank. When I did the math (miles divided by number of gallons), it came out to 27.2. Much better. It's not the first time. It's usually not that high, but higher than indicated.

 

[Sailingeric]

I think you need to track it several tanks since sometimes you might put a little more or a little less fuel on each fill up if you are one to round up the price once the pump clicks. I have not followed mine to much but my 2012 Mazda 3 was fairly close if I did a multi tank average.

 

[BlueReflex]

I'll do that.

 

[craigo]

"Real" MPG is hard to measure because you're relying on the gas pump which is likely to be not perfectly accurate. The car gets it data straight from the ECU and is likely to be very accurate.

 

[Puyapim]

I have found my calculations from filling up and the cars computer numbers to be 0.3-0.5mpg different which I think is pretty good. I never top off after the nozzle clicks off when fueling.

 

[murky]

"Real" MPG is hard to measure because you're relying on the gas pump which is likely to be not perfectly accurate. The car gets it data straight from the ECU and is likely to be very accurate.

But what is the ECU measuring? If you use the same gas station and let the pump click off without topping up, and put in more than 10 gallons, I'll bet that is more accurate than the car. If you put in an extra quart of gas at about 30mpg and 300 miles instead of 300miles/10 gallons arriving at 30mpg you get 300 miles/10.25 or 29.3 mpg. In order to be off by 2mpg you'd need to overfill by almost 3/4 gallon.

 

[craigo]

But what is the ECU measuring? If you use the same gas station and let the pump click off without topping up, and put in more than 10 gallons, I'll bet that is more accurate than the car. If you put in an extra quart of gas at about 30mpg and 300 miles instead of 300miles/10 gallons arriving at 30mpg you get 300 miles/10.25 or 29.3 mpg. In order to be off by 2mpg you'd need to overfill by almost 3/4 gallon.

Most folks on this topic have found the ECU and traditional measurement to be pretty close. In cases where it was far off it seemed to be an anomaly that was hard to repeat - calculation error, pump issue or perhaps forgetting to reset the computer.

 

[GJ-Molestor]

I still think the most accurate way to check is note how many litres you put into the tank up until the first click, reset the trip computer and then observe how many KM you get VS how many litres consumed.

 

[7eregrine]

I used an app to track like 40 fill ups. It's cool because it tracks a lot of info. I used Feulio. But it was consistently within 1 mpg of the what the car said, so I stopped.

 

[shadonoz]

I have found my calculations from filling up and the cars computer numbers to be 0.3-0.5mpg different which I think is pretty good. I never top off after the nozzle clicks off when fueling.

Which is what your supposed to do [fwiw]. But different pumps deliver at different velocities, and will click off at different levels of fullness of the tank.
The only way to do it is to average multiple fill-ups. But then you get into different driving conditions, climate conditions, etc.
No matter what you do, it's an estimate, or an average incorporating multiple variables.

 

[Puyapim]

Which is what your supposed to do [fwiw]. But different pumps deliver at different velocities, and will click off at different levels of fullness of the tank.
The only way to do it is to average multiple fill-ups. But then you get into different driving conditions, climate conditions, etc.
No matter what you do, it's an estimate, or an average incorporating multiple variables.

True, and the best anyone can do is to stick with one method and use the numbers as a benchmark. If your car gets 28mpg consistently and all if a sudden gets 20, then maybe that’s a sign of trouble.

 

[yugrus]

Keep in mind that in some locations (Ontario, for example) the fuel at the pump is volume-adjusted to +15C, no matter what the ambient temp. The ECU does not take that into account.

 

[Moonlighter]

But what is the ECU measuring?.

We have the same issue on our boats running modern efi 4 stroke outboards. And knowing how much fuel used in our boats is far more important than in cars, there are no gas stations to pull into and top up out in the ocean, so we have to know that it is accurate.

And, the data derived from the outboard ecu’s is incredibly accurate, and has been proven to be so.

How does it do it?

In laymans terms, the ecu determines exactly how much fuel to inject based on a wide range of information it receives from engine sensors. These include air temperature, load, speed, accelarator position, exhaust gas oxygen levels, and so on. Too many parameters to list here.

So it is simple for the ecu to constantly record exactly how much fuel has been used by the engine, because it detremines how much to inject in the first place.

(This has a very vague relationship to your fuel gauge, and in fact, the fuel level as shown on your gauge is not used to calculate fuel economy. At any point in time, your cars system will know exactly how much fuel has been used since the last reset. A reset sets the fuel used figure to zero)

The only other parameter required to give us real time (instantaneous) fuel economy (mpg, km/litre or L/100kms) readout is speed. (Speed of course also determines distance and distance allows an AVERAGE fuel economy figure to be calculated over a certain distance)

There are two potential sources for speed data that the car’s onboard systems can combine with fuel use data from the ecu for this purpose - the speedo, or the GPS.

I am not sure which speed source the Mazda system uses to calculate fuel economy, I suspect it might be speedo.

But I would also not put it past them to use the GPS data, which itself is incredibly accurate, to correct the speedo data so that it is “real” speed, and to then use that figure to calculate mpg. Wouldnt be diffiuclt to do at all.

As I said, we have found that the ecu derived fuel use data on our boats is very accurate indeed. As an example, i recently filled up my boats fuel tank and it took 33.3 litres to fill. The ecu said it had used 33.0 litres. That is a 1% variation. (0.3 litres = 300ml = about half a pint,or a large cupfull)

Now, that 1% variation could easily have been due to just how “full” I filled the tank, or even a slight variation in the angle of the gas station pavement resulting in the fullness of the tank being different. But over time, with repeated fills showing similar 1% plus or minus variations to that experience, the system accuracy has been well and truy proven.

 

[xtrailman]

My cars have both read about 2.5 to 2.6mpg UK high on the average read out, compared to brim to brim checks, I was made aware of this on another forum so checked for myself, not very happy about it to be honest.

So now when I look at the average read out I mentally subtract 2.5mpg.

 

[craigo]

For official numbers, EPA measures the weight of the fuel before and after testing. It's the most accurate way to compensate for all the variables.

 

[shadonoz]

For official numbers, EPA measures the weight of the fuel before and after testing. It's the most accurate way to compensate for all the variables.

It may be the most accurate method, but the EPA's number are reliably unreliable!

 

[SEOR MAZDA]

I always trust the computer to tell me how many mpg I get.
Doing the calculation manually is never accurate since some the pumps cut a different rate based on the foam the gas make when filling up.
If you want to be accurate doing it manually, fill the tank up to the neck, without spilling a drop, drive and repeat.
Please, do not forget to reset the computer after you fill your tank before driving away.[emoji3]

 

[erhayes]

I've compared my hand calculation vs the dash readout many times and the results or so close that I only use the Dash readout now.

 

[Jedi54]

I used an app to track like 40 fill ups. It's cool because it tracks a lot of info. I used Feulio. But it was consistently within 1 mpg of the what the car said, so I stopped.

+1 for Fuelio
excellent app that calculates real mpg, shows graphs on averages, costs, # of fill ups, etc.

I use it for my Daily Driver (non-CX5) and find that it the real time numbers are consistently lower than what my car is telling me I'm averaging.

 

[craigo]

We have the same issue on our boats running modern efi 4 stroke outboards. And knowing how much fuel used in our boats is far more important than in cars, there are no gas stations to pull into and top up out in the ocean, so we have to know that it is accurate.

And, the data derived from the outboard ecus is incredibly accurate, and has been proven to be so.

How does it do it?

In laymans terms, the ecu determines exactly how much fuel to inject based on a wide range of information it receives from engine sensors. These include air temperature, load, speed, accelarator position, exhaust gas oxygen levels, and so on. Too many parameters to list here.

So it is simple for the ecu to constantly record exactly how much fuel has been used by the engine, because it detremines how much to inject in the first place.

(This has a very vague relationship to your fuel gauge, and in fact, the fuel level as shown on your gauge is not used to calculate fuel economy. At any point in time, your cars system will know exactly how much fuel has been used since the last reset. A reset sets the fuel used figure to zero)

The only other parameter required to give us real time (instantaneous) fuel economy (mpg, km/litre or L/100kms) readout is speed. (Speed of course also determines distance and distance allows an AVERAGE fuel economy figure to be calculated over a certain distance)

There are two potential sources for speed data that the cars onboard systems can combine with fuel use data from the ecu for this purpose - the speedo, or the GPS.

I am not sure which speed source the Mazda system uses to calculate fuel economy, I suspect it might be speedo.

But I would also not put it past them to use the GPS data, which itself is incredibly accurate, to correct the speedo data so that it is real speed, and to then use that figure to calculate mpg. Wouldnt be diffiuclt to do at all.

As I said, we have found that the ecu derived fuel use data on our boats is very accurate indeed. As an example, i recently filled up my boats fuel tank and it took 33.3 litres to fill. The ecu said it had used 33.0 litres. That is a 1% variation. (0.3 litres = 300ml = about half a pint,or a large cupfull)

Now, that 1% variation could easily have been due to just how full I filled the tank, or even a slight variation in the angle of the gas station pavement resulting in the fullness of the tank being different. But over time, with repeated fills showing similar 1% plus or minus variations to that experience, the system accuracy has been well and truy proven.

ECU has to decide how long to keep the injectors open by knowing how much they are putting out. The injector specs are known, but a dirty injector can throw it off - the ECU will adjust a trim variable to compensate if it detects it (which it should these days). Should be very accurate on a modern system... that programming is critical to good MPG