Poor fuel economy in my 2009

Your car has a flexible has tank we cal l a bladder. Do a search for bladder and you'll find a wealth of information.

TheShort of it is you cannot count on manual calculations like in a traditional non- flexible tank because capacity changes with temperature. You must record many manual calculations and average them out over many many tank refills. Thankfully others have done this for you and have concluded the MFD readout is pretty accurate but read approx. 2-3% high. This is better then the GENIII which seems to be between 4% and 6% high. The GENiii doesn't have a bladder.

 

F8L's advice is spot on. Also, the "guess gauge" is pretty non-linear and the # of miles before the 1st pip vanishes can vary a LOT. It could be very few miles or as many as 150+ (esp. if the gas tank was run down a bit, then topped off while reading full).

 

I'm so glad I wrote this post for occasions like this Nothing's wrong, you haven't lost any mpg. Your display is close to accurate, about 2 mpg too generous. You can only calculate manually and average over time, because:

Mike with a Prius: Prius gas bladder

 

I am experiencing the same problem as the OP, although not as bad. I'm at about 36 mpg, according to my calculations, while the MFD is showing 46 mpg. So basically, I will sometime get the math to work out to a high enough mpg which will mean I'm within 2-3 MPG of what the MFD is showing?

 

Yes. Keep recording data until you have at least 10 refills but preferably 30+. You'll see the discrepancy reduced with each new data point.

 

And one thing that new owers really must check is tire pressures

The Prius is very suscepible to low pressures. Check the correct psi on the door frame and make sure your tires are at this level.

Some members on here pump a couple psi over these recommended psi's to get even better mpg's but be aware that this could affect grip.

Enjoy your new ride

 

You need to average out both over time in a spreadsheet. Sometimes, you will find the manual calculation is much lower than the MFD and other times, the opposite.

For me, the averages of each are within 1 mpg of each other.

 

that is great news, I was worried there for a bit. I've only had the car a week, so I will give it some more time to even out.

 

Don't worry about anything Prius related until you check PriusChat. The gurus like Cwerdna, Patrick Wong et al. will set you straight and likely save you money too!

 

In my 2009, if you do not reset the MFD, only the miles goes back to zero, the mileage continues to average. So you may be seeing the effects of previous tanks.

Push the Reset button yourself each time you fill up.

 

Good point. I always do. I believe the above behavior's the same on the 06 as I've a few times accidentally driven a bit w/o resetting.

It may have been different on years prior to 06.

 

Aye, the 2004-2005 Prius automatically resets the gauge when you refill more than a few gallons.

 

So you are at zero refills? You can't even calculate a crude mpg before the first refill, no gas gauge on any car I've ever owned was accurate enough for this.

 

To get as much into the tank as you can when you fill up, fill slowly...let the bladder expand.

I did 3 checks on the mfd mileage display in the first months I owned it by letting the avg consumtion accumulate through 10 fillups each time, then compared to a manual calculation of total litres pumped and total km driven. The reported averages were very close to actual... within the accuracy of the measurements.

 

No, I've refilled 3 times. I drive a lot.

 

The display is always pretty nearly correct. After 10 or 20 tankfulls the two numbers should approach each other closely, modulo any inaccuracy in the MFD and in the gas pumps you use.

 

Sorry, I confused you with the OP.

Something is fishy, the spread between displayed and computed mpg should not be as large as 10 mpg after several refills unless they were short tanks. Do you have a fuel log to share with us?

 

Since you say that your "averaged MFD MPG since then" is 49.9 we shall assume that you hit the reset button after your fillup rather than include anything the previous driver had accomplished.

I would suggest that you use up an entire tank until the last flashing pip. When the last flashing pip shows up, fill your car up fully. Now even in Chicago in mid April you should be able to get about/more than 8 gallons in the tank. This way you will know that you have at least 8 gallons of fuel and likely more. See how far that full tank will take you. Only refill when you're at the last pip again. Calculate your mileage and post your results.

The idea is that even with the bladder you can approximately guess how much fuel you have in the tank: if the full tank takes you 300 miles and you only had 8 gallons that would equal 37.5 mpg. If the full tank takes you 300 miles and you had 11 gallons in the full tank that would be 27.3 mpg. Your MFD mpg should be close to that figure.

The reason this sort of thing would be informative would be because your report of calculated vs displayed mileage includes such an unusual variation.

Please do not run out of gas. If you run out of gas do NOT use your HV battery for the nearest gas station, depleting it's charge below 40% (~1 purple bar in the MFD) may permanently damage the battery