MPG Accuracy Discussion

LOL It is not that I have "extreme views" to understate my mpg. In fact, I am a hypermiler and the higher the mpg is, the happier I am.

Last night I obtained 80 mpg with SOC unchanged in a 15-mile freeway trip. Was this really 80 mpg or 75 mpg? I would like to think it is 80 mpg, but I have had a hard time justifying these numbers with my pump and watt–hour-meter fills.

As I said, the speedometer has exactly a 2-MPH error; although, that seems intentional. Perhaps the fuel meter has a similar error and the two add up, resulting in a 7% error (~ 5 mpg) in mpg.

 

^^^ My opinion is that when they tested the car to get their EPA numbers, they used an average driver and not a hypermiler. So when the public buys the car and drives it, the numbers are realistically ... average. Everybody here knows a hypermiler can wring out better numbers, but they don't drive like the average person.

 

The testing is done to a set speed and acceleration profile on a dynamometer, with expert drivers following a computer screen telling them exactly where they should be.

These results are the CAFE figures, which are far higher than what the EPA eventually posts anymore. The EPA originally publicized the CAFE figures, back in the 1970s, but started discounting them in the 1980s to better match what average drivers really get. The amount of discount has increased several times since then, as higher horsepowers and increasing speed limits and increasingly aggressive 'standard' driving behaviors (and even increasing use of AC in warming climates) caused an ever-widening gap between those CAFE test results and real-world results.

If the EPA used actual 'average drivers', the test results would become less repeatable. Expert drivers following the CAFE profiles, combined with set discount formulas, produce more consistent results.

The precise test profiles are shown here:
https://www.fueleconomy.gov/feg/fe_test_schedules.shtml

 

In an old article on the this topic, it was stated that the calculation for the displayed mpg was based upon the fuel being injected into one cylinder. The computer doesn't actually know the volume injected. It knows the pressure in the fuel rail, injector pore size, and time it was open for. Differences in the pressure along the rail, and variances in pores sizes, means each injector isn't squirting the exact same amount of fuel in every time. But watching one cylinder means the system assumes that they are all the same, which opens an avenue for error.

Then while laws govern the accuracy of odometers and speedometers, they don't for these displays. The manufacturer is free to have the display to be more optimistic than what the computer actually calculates.It is only the concern of negative backlash to keep this BSing in check.

All my cars' displays have been optimistic; none have been under reported in comparison to what multiple tank average calculations have returned. Well, the Sonic may have under reported, but it ignored EOCed miles for the calculation.

 

These new in like 1993 instant mpg readouts measure to .1 mpg. The accuracy of fuel measurement has to be there to do that, there isn’t any faking.

It’s sort of like me trying to look at the second hand of my watch, the odometer, and holding steady 60 to measure 60 mph, compared to someone using the gps.

As can be seen I can’t figure out how to undo the underlining yet.

I think I figured how to undo underlining finally on my ipad. At least I did something and it worked. Is there a number readout on the Prime for real time mpg? That’s easy to see?

 

Just because they know how to make it very accurate, doesn't mean any government regulation requires them to do so. A bit of "optimism" makes unknowing drivers -- which is most of them -- feel better, which is advantageous to their marketing efforts. This is only one of many "customer expectation management" features (a.k.a. "lies") designed into many cars, to either make drivers feel better, or to reduce customer service calls and complaints.

And no, the ECUs don't really need that degree of accuracy. They use feedback from the oxygen sensors to trim the injection quantities and adjust for initial errors and offsets. Even if fuel quantity was precisely known, this feedback is absolutely mandatory to properly adjust for differing fuel blends and ethanol concentrations.

ScanGauge-IIs don't start out that accurate, but must be user-calibrated by feedback of the gallons added during refueling. The airflow readings they get over OBDII ports just don't have the initial accuracy.

On a laptop, re-edit the post and look for the underline icon, to toggle on or off for selected strings:

 

I’ve done similar with our 3rd gen, and “error” settled down to 7.5%. There’s some up and down vagaries, but the average remains steady. Like you, I haven’t mentioned to which side the “error” lies. It’s understood…

A work-around that works for me: if my (metric) trip meter is displaying say 4.6 (liters per 100 kms), adding 0.3 will get you pretty close.

It seems a stretch, to account for an rock-steady, positive bias as an accuracy limit?

I’ve heard 5th gen dash displayed mpg is finally accurate?

Our 06 Honda Civic Hybrid’s (liters per 100 kms) display was commendably accurate, one of a very short list of positives.

 

I think if Ford 20 years ago put it in writing in the owners manual to rely on the display not pump calculated, they wouldn’t have rigged the display. Display would just be rigged. In writing, published to every owner, is strong evidence their display is accurate. They had some reasons listed but don’t have the manual. Probably pump shut off, pump and gas tank temps, odometer reading and tire wear, things like that.
Thanks for helping with my modern machine disability.

 

I thought this was about Toyota and Prius, not Ford.

 

Ford was probably fielding too many complaints about differences between the display and what people were calculating from just one tank. The hand method has its own sources of variance that leave it more suitable for long term averages. Plus, when it comes to costs per mile, it is the one actually using the amount of paid for gas added to the tank.

 

Well, it has a title.

 

With two generations of Auris Hybrid (gen 3 powertrain) i ended up with the same empirical conclusion, about 7% error, the same (intentional) error of the speedometer. Same with the electric consumption of my Tesla M3 LR (different value but same pattern).

 

I have had my 2023 Prius LE all-wheel-drive for a month and a week and so far the displayed mpg is fairly close to my miles driven and gallons to fill up tracking on my Excel spreadsheet. My total average odometer mpg is about 52.8 and I am getting an average of about 52.6 on my Excel spreadsheet. Granted, there are only four entries so far.