Do you LIE about your mileage?

Everything I say is a lie, but this time I'm telling the truth.

(from the original Star Trek).

Or, how to put an old design computer into a loop (newer chips have built in divide by zero protection).

I tell the truth. It can get complicated, because the truth is complicated. For example, mileage when? It varies by time of year, by speed, by terrain. Average? Best? Worst?

 

Those people who compare the mfd mileage to calculated values.... What distance are you using? The odometer or gps? If the odometer, then that'll be a bit out by things like tire wear etc. And I don't know if it's true everywhere, but around here, the volume of gasoline dispensed at the pump is volume corrected to 15C. I wonder if the metering of the fuel flow by the system in the Prius adjusts for temperature differences in fuel density. These could partly explain a few percent difference, I suspect.

 

I use 2 programs on my iPhone and have used a few different sites online to calculate. My MFD is always bang on with the programs I've used to calculate. It's always between 4.4L/100km and now that it's getting cold 4.9L/100km.
I usually just say 50 US mpg.

 

I use my odometer, which appears to be reading 0.5% low compared to the mileposts on a 100+ mile stretch of a regional freeway. (I tried for 140 miles, but there was an unnoticed but blatantly obvious discontinuity somewhere between 101 and 120).

Based on other experience, I would expect my GPS trip measurements to have a much larger error, and there is no way that I could get whole-life GPS trip tracking of my vehicle.

As for tire wear, will it be significant? Other discussions have suggested that rolling circumference is more related to the internal belt length than to the actual tread circumference. As the tread wears down, revolutions per mile may change much less than you expect.

 

Then what, seal the results

 

I track both MFD mileage and calculated mileage, and calculated seems to be much more erratic than MFD. I attribute that to the bladder, high this tank, low the next, high after that.

I also track a four-tank rolling calculated average, which actually tracks closer to the MFD than to the calculated. this, I think, lends credence to my assumption that the erratic nature of the calculated mileage is due to the bladder, and averages out over a few tanks.

Here is a chart of MFD, calculated and rolling over the life of the vehicle (~4 years, it's a 2005 bought in september 2005).



all that said, I realize I haven't answered the question. I don't lie. I quote the calculated mileage over the life of the vehicle, 46.42 mpg, or 5.07 l/100km.

A quick comparison of MFD versus calculated for the life of the vehicle shows the MFD is high by ~2.64%...

 

It has zero impact since the MFD is using the odometer as well. Relative to one another the difference would be zero if the quantity of gas the MFD believed was consumed agreed with the pump.

This is one of the effects that I believe is occurring, particularly since I've seen the worst departure in cold weather. Gas in the underground storage tank will be warmer than the gas in my tank after a few hours. I doubt there is temperature measurement done at the fuel injectors and therefor there is likely no temperature compensation for it. But perhaps they have some clever corelation to account for this.

Another aspect is that there is some finite quantity of unmetered fuel entering via the vapor recovery system. I suspect this is quite low (probably in the range of .1% or less) but have found no figures for it.

Yet another factor is fuel that never makes it into the tank...a few drops in the nozzle, vapor burp, etc. This would be even smaller but yet another provable departure.

 

I report the actual mileage (53.4) that I calculate. So far the MFD is about 4.5% too high on my GIII.

 

1. Yes

2. I usually try to give the calculated mpg since most people act like they have a hard time believing it anyway. So, why give them something else to doubt.

After eight tank refills, my Gen III MFD is displaying high by an average of about 3.5 MPG or about 7%.

I have tried to be very consistent by trying to go to the same service station and even to the same pump. I try to repeat the refills each time in order to try for the most consistency.

 

1. No. I have more important things to do than worry about 5% differences in my gas mileage. I do check - when there are big changes from this, it usually means my tires are unenvenly inflated or underinflated (TPMS is useless and nothing but another expensive hassle when you have more than 1 set of rims).

2. I get 43-44mpg (higher when it's warm). It is what it is.