MPG On Screen vs Reality

[Guardian_Bob]

But it doesn't. You're missing the point. We all know that the car underreports how much fuel it has burned. Every car does this. But this car also underreports how many miles it has traveled. If you're going to correct the fuel use by calculating based on gallons at the pump, you should also correct the miles inaccuracy. It isn't fair to correct one inaccuracy but not the other in your calculations. Saying that the trip computer MPG is off by 5% is not accurate. Since MPG is a factor of two components (miles & gallons used) it isn't truthful to adjust the gallons for the inaccuracy of the dash display without adjusting the miles as well for this same purpose.

Actually, you missed mine.

 

Why should the car ever under report the fuel consumption by 5%? Mileage can be affected easily by tire size, inflation, etc. These items cannot be corrected by the computer. Fuel consumption doesn't have that issue. You can correct for time (evaporation?), temperature, etc. X gallons go in, X gallons go out. You should always get an accurate number for that.

[hybridbear]

Actually, you missed mine.

 

Why should the car ever under report the fuel consumption by 5%? Mileage can be affected easily by tire size, inflation, etc. These items cannot be corrected by the computer. Fuel consumption doesn't have that issue. You can correct for time (evaporation?), temperature, etc. X gallons go in, X gallons go out. You should always get an accurate number for that.

Every car is inaccurate in this respect. It would take sophisticated equipment that costs thousands of dollars to get a reading with a high confidence level. Instead we get a reading with a 95% confidence level. In statistics a 95% confidence level is considered to be very accurate.

[Guardian_Bob]

Every car is inaccurate in this respect. It would take sophisticated equipment that costs thousands of dollars to get a reading with a high confidence level. Instead we get a reading with a 95% confidence level. In statistics a 95% confidence level is considered to be very accurate.

95% confidence does not imply 95% accuracy. Be careful conflating the two. They mean different things.

 

What your talking about is confidence intervals for statistically significant behavior. How likely cause A is to result in effect B.

 

And to your point, it wouldn't take thousands of dollars of expensive equipment, you could simply measure the weight of the gas tank. I suspect Ford is using a flow meter, but that doesn't mean the calculations can't be done to adjust for the variables.

 

I pointed this out to Ford service, they are going to have the CSM for the region contact me. It is worth having others check this out as well. This is one small thing that can have a big effect. It also could explain why some cars appear to be better in MPG than others.

[acdii]

It is all estimated. Fuel used is based on flow rates and injector timing. If there is any discrepancy, it is in this area of the calculations. It may or may not account for fuel returned to the tank after each cycle. What it estimates, and what most of us see in reality is more fuel is actually sent through the injectors, than the car is estimating. The only way to correct this is to calibrate the trip summary with the gallons used like you do with a Scangauge. If Ford were able to allow us to correct the fuel used on each trip, then you can see better accuracy in both gallons used and MPG.

[Charrua]

But it doesn't. You're missing the point. We all know that the car underreports how much fuel it has burned. Every car does this. But this car also underreports how many miles it has traveled. If you're going to correct the fuel use by calculating based on gallons at the pump, you should also correct the miles inaccuracy. It isn't fair to correct one inaccuracy but not the other in your calculations. Saying that the trip computer MPG is off by 5% is not accurate. Since MPG is a factor of two components (miles & gallons used) it isn't truthful to adjust the gallons for the inaccuracy of the dash display without adjusting the miles as well for this same purpose.

I was not aware that it was well established that the FFH under-reports miles.

I have not done a careful study on mine but I do know that the MPH reported by my FFH and by my GPS (going at highway speeds) are within a fraction of a MPH of each other. Would this not indicate that on my vehicle the miles are not under-reported, just the gallons?

[Charrua]

It is all estimated. Fuel used is based on flow rates and injector timing. If there is any discrepancy, it is in this area of the calculations. It may or may not account for fuel returned to the tank after each cycle. What it estimates, and what most of us see in reality is more fuel is actually sent through the injectors, than the car is estimating. The only way to correct this is to calibrate the trip summary with the gallons used like you do with a Scangauge. If Ford were able to allow us to correct the fuel used on each trip, then you can see better accuracy in both gallons used and MPG.

It's my personal opinion that Ford could easily make the fuel usage calculation a lot more accurate. Every FFH seems to be skewed by 3 or 4 percent. The problem is that they have no motivation to do so. The skewing is in their favor and there are no regulations that require them to have a certain degree of accuracy (unlike gas station pumps).

[hybridbear]

I guess I was misapplying my terms. Sorry.

 

I guess in the end I don't find the inaccuracy to be that egregious. Every vehicle is inaccurate. On a 15 MPG car a 5% inaccuracy is less than 1 MPG. This appears much closer if you calculate 15.0 when your display says 15.75. Very few owners are going to notice or complain about such a discrepancy when it's the same % and is actually much worse from a gallon difference over a set number of miles. We found our Honda Accord Hybrid to consistent 3 MPG off which was closer to 10% since it was about a 30 MPG car. The Prius has been commonly noted to be 6-7% off. This is worse than the FFH discrepancy.

[lolder]

Prius owners report +6 to 8% too high mpg's from the computer.

Edited October 29, 2013 by lolder

[acdii]

Hey I had an F350 that told me I was getting 36 MPG. I dont put much faith in the dash gauges, the proof is in the puttin in the tank. None of hte cars I owned were spot on accurate, they had a variance between 5 and 15% depending on how much gas I squeezed in. They dont take into consideration air temps, and evaporation.