Computed Average Does Not Match Actual Average

http://priuschat.com/forums/gen-ii-prius-fuel-economy/72358-mpg-inaccuracy-problem-not.html

I had exactly the same question, recently, but mine displays 10mpg LESS than what I compute. Since I don't have the GPS, I can't compare the speedometer to the readout, but if yours is off several miles per hour, that may indeed be a source of error.

Chuck

 

ound:

I'm sorry for laughing, but this question is one of the most commonly discussed issues on this forum. There must be hundreds of posts dealing with this exact issue.

Anyway, back to the question. The Gen II North American Prius has a fuel bladder in the tank. There are many good reasons for the bladder, but the offshoot is that the usable tank capacity is highly variable. There is no way to accurately calculate mileage on a tank by tank basis. The fuel bladder introduces too much uncertainty.

If you want to do manual mileage calculation, the only way to do them accurately is over a large number of tanks. The tank to tank error will average out over a large number of tanks.

Tom

 

yep! Bladder problems again LOL. The GPS trick is probabbly the best way over a long distance to do a cross check. Or 32 fill ups and then do a running average. I have been 1.5 gallons off on fill ups.

 

For example after 13 tanks my calculated and MFD's lifetime MPGs differ by 2%. For the first two tanks the difference was 4%.

 

Exactly, and incorrect by a large margin. You can easily see a two gallon variation on a fill up, which is a lot for a tank that only holds ten or eleven usable gallons.

Tom

 

Or a 4.5 gallon variation...the last tank in ours. The bladder/vent system refused to allow more into the tank in 20 F weather. Level was reading about right at 5 pips after the abortive fill.

That's twice...when it gets to three I'm going to get medieval on it.

 

I've had calculated numbers, over the last 8 fill ups, that range from 40.1 to 62mpg. My displayed mpg was 44-48mpg.

One of these two is wrong.

Given the enormous variation in calculated mpg, I expect that it's an inconsistent fill level, resulting in an inconsistent calculated mpg.

Chuck

 

Inconsistent fill and the bladder problem will create a discrepancy on individual tanks, but not for the long-term average (sorry Tom). Generally I notice in the fall the bladder is shrinking and my computed values beat the screen MPG, whereas in the spring the bladder expands and now it takes a little more gas on each fillup. But overall it averages out pretty close (I have the spreadsheet to show this, being an engineer).

One thing that should be asked - do you have the original tires on the car? If they are a different size, it could throw off your computed value since your miles traveled won't be correct. I'm assuming the value on the MFD uses the flow rate or time duration the fuel injectors are on for each piston fire. If that's how the Prius computes it, maybe old/dirty injectors could be the problem - burning richer/leaner than expected.

One thing I do is always try to fill at the same station, even the same pump that provides reasonable numbers. You never know if a gas pump is delivering exactly what it says it delivers. By using the same pump at least I'll have trends I can follow even if the baseline (DC offset in electrical terms) is not correct, but like I say, for me the computed average and MFD average is pretty close over the long term.

 

I'd blame the shutoff trigger on the pump nozzle before the Prius bladder in this case.

 

I wouldn't. I've never had this happen on any other vehicle. It's definitely the car.

 

I've had it happen several times on two other cars, but the Prius is more prone to it.

Tom