My Prius reporting wrong MPG

The big variable is how much gas you put into the tank. You may have read about the bladder the Prius has for a "tank," and the varying amounts of gas you can get into it each time.

If it happens to click off early when there is still room for another couple of gallons in the tank, your mileage will look better than it should. Likewise, if you are able to squeeze more than normal into the tank your calculated mileage will be reduced.

Over time these variations should even out, but I trust the computer-generated MPG figure on a tank-by-tank basis.

OB

 

Could you clarify how you are determining actual MPG at the 100 and 300 mile intervals?

 

Lets assume for a moment that the MFD is accurate.

For your 100 mile trip, the MFD reports that you used 2.5 gallons (100/40) where you reported using 2 gallons.

For your 300 mile trip, your MFD reports 6.12 gallons used while you report 7.69 gallons.

But lets add them up.
MFD says you used 8.62 gallons while you filled a total of 9.6 gallons.
It is quite possible you could have a difference in fill by as much as a gallon.

 

This goes against all things Green, but...

I cram the tank full of gas in order to be sure it's full. I stop when I see gas left near the top. I then drive until my MFD display shows I've driven 10 times the current MPG average, then fill up again, hopefully using 10 gallons.

I've never varied more than a half-gallon when I do this, and it's always been more gas than I expected to pump, not less. Given bladder stiffness and other variables, I don't think I'm ever off more than 5%. As I've got no reason to keep a journal of MPGs, I only check this occasionally.

When it's almost dead-on accurate, which is often, I'm happy...

 

Please tell us when your emissions sensor fails, because it eventually will.

It was designed with the intention of sensing vapor, not to be submersed in liquid.

My parent's purchased a rental Corolla earlier this year. Their sensor failed at just 31,500 miles. Overfilling the gas tank (which floods the emissions canister and all the piping) was concluded to be the most likely cause.

 

I'll be sure to write.

36,406 miles as of this morning. Surely to fail sometime this afternoon...

 

I used to have big variations as much as +7 MPG one tank, and -7 MPG next tank compared to the MFD calculated one.

I never overfill the tank, always let it go on the first click.

Lately the variations have gotten a lot better, probably because I've settled into a routine, and have started going to the same gas station and try to use the same pump every time. Last 3 tanks have been within 1 MPG of the computer.

As long as you stay consistent in the way you fill up at the pump, it should eventualy even out to what the computer tells you.

And for the reason that John posted above, I'm not going to overfill the tank. I think that emissions and the health of the car are more important than outright range. I put in 8-9 gallons every time, and only get about 450-480 miles to a tank, but that's fine by me.

 

Like MikePaul, I try to fill my tank full, at least when I have time.
But I generally stop when the noise changes.

If the car is designed right it shouuld never fill the canister. There is a valve to prevent that in the new tank fix.

Anyway, there should be no risk at all if you stop at the noise change, which should be where the gas gets to the top of the tank and just starts going up into the filler tube.

Even when I got it up to wher I saw gas in the neck my scanner has never reported a purge, so I have to assume my tank valve is working just fine and protecting the sensors.

 

100 miles?? 300 miles??

look at my LT stats that is for 18,000 miles. look at the difference there....

my single tanks or "short" bursts varied my nearly as much as yours. last year i was in a monumental traffic jam. averaged over 85 mpg (according to MFD which is easy to figure if you have a start and end point)

but even if you did fill up after 100 miles. unfortunately, the tank error will not be a percentage of the fill, it will be a roundabout average of the emptyness of the tank.

so the average "emptyness" of your tank is say a gallon +/- ½ gal.

you fill up after 100 miles and add anywhere from 1.5 to 2.5 gals if averaging 50 mpg. that leaves a possible range of 40-66.6 mpg. so you know you will fall in that range somewhere. now we could use a standard bell curve and narrow that range a bit but you get the point

so now we fill up at 300 miles with same MFD average of 50 mpg. the percentage of "emptyness" may change but the volume does not. so now we will have a range of 5.5-6.5 gallons for an average of 46.2-54.5 mpg. as you can see, this still isnt gonna help you perfect your driving style since many single things you can do may only give you a mile per gallon.

in short, forget the short term mileage. unless you are doing very accurate fills after every one of these fills, you are wasting your time for the most part. use the MFD readings for comparisons yes, but overfilling your tank to increase the accuracy of your mileage readings is imho, a HUGE mistake. the gains you get (not sure what they would be to be honest with ya) aint worth the possible penalties.

keep in mind that full tanks of gas add weight and will cause a drop in mileage

 

I typically fill up with 1-3 bars left on the guess gauge. And I generall add fuel once after shut off, but it usually shuts off immediatelly anyway. Last time I actually added an extra gallon, just because I wanted to see how much I could really get in there.

Anyway, I have a pocket pc program that I plug everything into. And over the 3500 miles I've drive, my calculated average is 49 mpg. I don't recall the MFD average off hand, but it's pretty close. On individual fill ups, I've been as low as 44 and as high as 80! So don't go by the individual fill up - keep a running total.

 

Ok, so I've checked my records. I haven't downloaded recently, so I only have my first 2000 miles.

My MFD average is 50.47
My Calculated average is 50.04

FYI - The program I use is Two Peaks Personal Vehicle Manager