How accurate is the gas mileage as reported by the computer?

This very question has been addressed numerous times in other threads, but in short, here is your answer:

The MFD fuel calculations are fairly accurate. Long term testing usually shows the computer calculated mileage to be optimistic by 1% to 3%.

Manually calculated mileage is almost always off over a limited number of tank-fulls. This is because the Gen II Prius contains a fuel bladder (NA models), and the fuel bladder introduces volumetric uncertainty. You never really know how much fuel is burned for each tank. The only way to have meaningful manual manual calculations of mileage is to total the mileage over a large number of tanks. This takes the volumetric uncertainty and spreads it out.

Tom

 

No, not really. I consider 10 a fairly good starting number. However, if you correctly stated your calculating procedure in the OP, it won't matter how many tanks you use, since you are doing the calculation on a tank by tank basis. You need to total up the gallons and miles from all of the tanks collectively, and then divide it out.

Tom

 

What is your typical MPG? For example 3 MPG of 55 MPG is only about 5% so it's not too far off.

 

The Gen I and II -- all models up to and including 2009 --
have the fuel tank bladder, the 2010 Gen III does not.

Far more than you ever wanted to know about the Gen I/II fuel system
here.

 

For the most part, this is not a good idea. Because the bladder has a
variable volume, apparently it gets stiffer in colder temps, you can
never tell exactly how much fuel is in the tank. Likewise and more
importantly, you can't estimate with reliable accuracy how much
fuel/miles remaining that you have at the bottom of the "tank." The
"guess gauge," gas gauge is little help.
FWIW, it is recommended that you refuel the Gen II at two pips.

Running out of gas in the Gen II is a real no-no.
Lots of threads on the possible downward spiral of connected issues
can be found with the search function.

 

This will mostly work, but not as well as totaling. Because of the variation in tank sizes, the tanks that you average will not be the same size, so by averaging you are compounding the error component by giving equal weighting to unequal parts.

All Gen I and Gen II Prius for the North American market have the fuel tank bladder. Gen II for other parts of the world do not. The Gen III models do not have it.

Tom

 

can my gen II be converted to a bladder-less tank?

 

Practically speaking, no. Since it is tied into the emission control system, there is a lot more to it than simply switching the old tank for a new one. I'm sure it could be done, but not for a reasonable amount of money.

Tom

 

My MFD mpg vs. Calculated (Just did them tonight)

Actual Lifetime 46.2
MFD Lifetime 47.1

Readout error 1.9%

...............ODO...Trip....Gals........Actual MPG........ MFD MPG
03/25/10..60016..358....7.39............48.4 ...................48.7
04/02/10..60387..371....7.98............46.5 ...................48.6
04/06/10..60746..359....7.77............46.2 ...................47.1
04/09/10..61072..326....6.20............52.6 ...................48.6
04/13/10..61384..312....7.17............43.5 ...................48.8
04/15/10..61728..344....8.06............42.7 ...................47.8
04/18/10..62119..391....8.14............48.1 ...................45.1
04/20/10..62476..357....7.34............48.6 ...................49.1
04/23/10..62864..388....8.37............46.4 ...................47.5
04/26/10..63228..364....8.28............44.0 ...................44.8
04/29/10..63586..358....7.76............46.2 ...................47.7
05/02/10..63976..390....8.29............47.0 ...................46.4
05/05/10..64375..399....8.47............47.1 ...................49.2
05/09/10..64749..374....8.70............43.0 ...................44.1
05/13/10..65093..344....7.00............49.1 ...................45.0
05/15/10..65466..373....7.85............47.5 ...................46.5
05/18/10..65788..322....7.85............41.0 ...................45.8


Info about my drive.
90% Highway 10% City
120 Miles Per Day
Avg Highway speed 65-70
Avg City Speed 25-30

Was hoping for higher than 46 mpg, but it's a LOT better than low 20's I was getting with my last vehicle.

Thanks!

 

I find that doubtful. My averages of MFD vs. calculated are pretty close but I am definitely NOT seeing your friend's behavior on my 06.

On relatively large fills (7+ gallons), they could be anywhere from within 0.1 or 0.2 to ~3 mpg off. On really short fills, say <2.2 gallons, where the margin of error is huge, I see one that's 16 mpg off. Hence, I almost never do short fills.