How can U be Sure?

[drazoth]

Hi Fellow Prius owners,

I just came home with a 2008 yesterday manily for the mileage as I am commuting a lot and had a Corolla.
I'm now concerned that the computer read out of the gas mileage is not actual, but there is no accurate way to determine this due to the flex/ rubber tank.
Any suggestions?

Much Thanks,
Matthew

[tasker109]

we just took a trip to ohio from the st louis area. 428 each way.......we kept track of the gas we put in and set the trip odometer. when we got home, we did the math on a calculator and came up with exactly what the car computer read.........48.6 mpg.

[drazoth]

Did you account for the gas that was in the tank when you were filling? (how did you estimate that?)
What is your capacity for your tank?

[Turbogizzmo]

Umm correct me if i am wrong, but all you do is fill up the car to full (nice warm day) reset the odometer drive around for 100+ miles, go back put gas in the car and look at how many gallons you had to put in to make it full again compared with you odometer reading....do some math....compare it to your car-puter and it should be close....

Of course there are going to variables so you might have to do this a few times or keep a log and compare numbers....

[drazoth]

Thanks. That is what I was confused about - the variation in tank size and capacity....

[richard schumacher]

One or a small number of fillups will not give an accurate MPG figure: the tank bladder has variable capacity so the amount you burned since the last fillup is usually not the same as the amount you put in at the current fillup. Long-term record keeping by many people reported in this forum shows that the MFD MPG value is almost always within one or two percent of the manual MPG value.

Interestingly those MFD MPG values are almost always larger than the manual MPG values. There are several possible explanations:
- the MFD exaggerates a little to make us feel better
- the typical gas station pump cheats a little to make more profit for the gas station
- some fraction of the fuel evaporates before it can be used

[a priori]

Folks & Fellow PriusChatters!!!!

This is one of the most commonly asked questions. A new thread starts on this topic way too often.

The short answer: The variation in available space due to expansion/contraction of the bladder means that there always will be a difference between the MFD mileage and the number you calculate. Even after 30 tanks, I still have variation.

Please start by reading the most recent thread on this topic (click here), then do a search or go to the Fuel Economy forum.

Best to all!

[Spectra]

Originally Posted by a priori

This is one of the most commonly asked questions. A new thread starts on this topic way too often.

hear hear !!

[JimN]

Originally Posted by drazoth

Hi Fellow Prius owners,

I just came home with a 2008 yesterday manily for the mileage as I am commuting a lot and had a Corolla.
I'm now concerned that the computer read out of the gas mileage is not actual, but there is no accurate way to determine this due to the flex/ rubber tank.
Any suggestions?

Much Thanks,
Matthew

The only time the Guess Gauge should be believed is when the last pip flashes.