The dash display which shows the trucks average MPG in my truck is consistently wrong by about 2 MPG.
For example the dash will read 16.7 MPG but when I calculate the mileage using miles driven (according to the odometer) vs gallons of fuel used from fill-up to fill-up I consistently get a figure closer to 14.5 MPG.
I have seen others comment on the inaccuracy of this device and was just wondering if anyone knows if it is possible to have this device calibrated to be more accurate?
Mine is usually within .5 MPG of actual mileage. I have an application on my phone that I use to enter data each time I fill, so I have quite a few data points to compare. I've noticed that it can vary more depending on how much time I spend "topping off" the tank. I can usually get 2 or more gallons of fuel in the tank after the pump shuts off if I keep pumping a quart or so each time the foam subsides. One time it was almost 4 gallons.
When doing hand calculation of MPG, I always spend the time to top the tank off until there is liquid fuel (not foam) visible all the way to the top. I then continue "topping off" until I can watch the fuel and see that it is no longer "settling".
Due to the way diesel fuel foams this "topping off" procedure may add 2 gallons one day and the next time it may add as many as 6 gallons or more. This is the only way I know to get anything even close to a valid feul usage on which to base MPG calculations.
When calculating this way my dash display is usually off by 2 to 2.5 MPG. This is based on a dash reading of around 16.5 MPG (sometimes as low as 16.3 and sometimes as high as 16.8). For these same comparisons the calculated value, based on (I believe) very accurate fuel usage and the trip odometer, would typically be around 14.5 MPG. These MPGs are typical for my normal daily driving which is a mix of city and highway driving. For pure highway driving, I get somewhat better MPG (18 to 19) on the display but the inaccuracy remains about the same 2 MPG so my highway MPGs average around 16.
I have seen many others on this forum comment on how inaccurate these displays are. In fact it is from these other posts on the subject that I got the term "lie-O-meter" used in the subject.
Anyway, I am still wondering if there is a calibration which can be done (either by myself or by the dealer) to improve the accuracy of the displayed value.
mine is usually right on about .5 mpg high but that is better than any other vehicle i have ever seen, i have a buddy that has a dodge 2500 05 it always says like 25 mpg and hes average around 18 hand calculated
Wow, you guys don't have much of a lie-o-meter. I always top off as well, and my lie-o-meter is consistently 7 mpg off. I'm usually in the 23-26 MPG range on the lie-o-meter when hand calculated is at 16-19 MPG.
So what do we attribute the error of mine to...my truck is bone stock.
I really do not understand how these things can be wrong unless the truck has a mod that would change the odometer such as tires, rear gears, tranmission (all of which can be adjusted for). As for fuel usage, the ECM should know exactly how much fuel has been used as it is managing the delivery, supposedly, very precisely. So with a very precise usage numbers the only other variable is the distance traveled. If the truck has stock drivetrain and factory equiped tires, how hard can it be to get an extremely accurate MPG calculation???
So can the same calibrations that are done to correct the odometer/speedometer when tire size is changed, be used to calibrate the MPD reading?
That could only be true if the calculation were using the fuel gauge to know how much fuel was being used. If in fact that is the case, I could see how the result would be wildly off since not only would the degree of full be a variable but also, fuel gauges are not all that accurate nor are they completely linear.
However mine is not is not wildly off...rather it is consistantly off.
If Ford is using the fuel guage for this calculation, I just have to ask why???
Again the ECM (or whatever is controlling the injector pulsing) is metering the fuel delivered to the cylinders extremely accurately. As I mentioned previously, with this extremely accurate fuel usage and a reasonably accurate distance traveled measure (from the odometer) they (Ford) should be able to calculate the actual MPGs very accurately.
I am math retarded (yeah I said retarded) and so can someone smarter than a 5th grader give me the formula for the hand calc part of this. I too, would like to see how far mine is off. I have a few mods as well and actually..think my odometer is off..but i want to know by how far off it is.
Thats it? what about tire size and all that geometry crap?? I have gone from stock 20's to 22's and my tire height is the same as the factory but the width is about .4 wider. just didnt know if there is some NASA exquation for that...if this is the case for my retarded and yet simple math..then too easy. thanks bullet
Believe me, I understand what you are saying. But, by your logic, there should be no reason for why the computer is wrong with a tune. If it knows how much fuel is flowing and how many miles are driven, it should be right on the money no matter what. I am not sure at how they are arriving at the MPG figures displayed on the Lie-o-meter, because it defies logic. I just know from experience that mine was always more accurate if I did not top the tank than if I did top the tank. You can't get a real reading hand calcing without opping the tank. I just know that running the same routes day after day what I get for MPG. If I do not top the tank the lie-o-meter reads lower.
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