2009 - 26 MPG but display says 44.6 - HELP!

I have a 2009 that just hit over 1000 miles and on my last tank I got to 380 miles and then the gas light came on and I filled it up for the 3rd time and my calculated was 45.76 and I didnt write it down but I my display averages between 45-46mpg but I only drive short distances. The farther distances you drive you will notice you will get better gas mileage because of the car warming up. Also the motor will get better MPG once it is broken in and the tire pressure will definitely help get better gas mileage.

 

So did you calculate the overall mileage by total miles driven divided by the gas you put in? Remember the first tank may not have been completely filled by the dealer. The more tanks in your calculation the more accurate it will be. Unlikely for the MFD display to be off by 20 MPG since the car knows how far it went and how much gas it injected into the engine with fairly good accuracy. Various threads on Priuschat have cited MFD accuracy to within one or two miles per gallon..

 

A couple of thoughts:

- Its almost impossible for the MPG gauge to be that far off. Its uses the firing data from the fuel injectors to estimate fuel flow, over a distance. If your injectors were that far off, your tuning would be so bad the car probably wouldn't run. The other possibility would be your speedo is off by a factor of 2, also very unlikely.

- Its very easy to see a large variation every time you fill the tank. The manual says the tank is 11.9 gallons, but that doesn't account for the flexible bladder in the tank to reduce evaporation. In reality most people get 9-10 gallons into an empty tank in warm weather, and as little as 6-8 gallons in cooler weather. This varies with temperature, bladder age, fill speed, and pump sensitivity.

- I'm having a little trouble making your math make sense. If you are getting >300 miles per tank, and you are calculating 26mpg, you would need to be getting >11.5 gallons into the tank on each fill up. As mentioned above, this would be pretty unusual. For the 44.6mpg to be correct, you would be putting in >6.7 gallons. That sounds a lot more likely. We'd need your actual numbers to check it.

- The first 2 pips coming off in 51 miles is unusual. I would think that would be consistent with a very "light" fill. Even on a light fill, the gauge will often go to 100%, as it doesn't know exactly where "full" is due to the bladder issues. It will then pretty quickly revise its estimate down as it figures out how much gas is really in the tank. Most people find on most fills that the gauge "pegs" off the top, with the first pip not coming off for 80-100+ miles. That would be more indicative of a good fill I would think.

Based on all that, it may just be that you're bladder is fairly stiff from a combination of being brand new, and the fall weather. That would cause it to shut off the pump early, resulting in a light fill, but still registering full on the gauge. Then the gauge would drop pretty fast as it realizes there is less gas in there than it thought. You should be seeing that in your calculation though, assuming you are using the actual gallons you put in and not just assuming the tank is a certain fixed size.

For a start I'd recommend filling at half speed to allow the bladder time to stretch. In general you want to avoid "topping off" as it can damage the vapor recovery system. I have to admit I often do fill to the second click though. Next make sure you are filling up on a level surface, there are inclination sensors that correct the fuel level which will get messed up if filling on an incline. Last, make sure your gas station is not ripping you off, and no one is stealing your gas!

Rob

 

Did you read the thread for newbies on getting good mileage? I found it great help and have been getting 45-55 for 30,000 miles average about 51.5 and I'm in Seattle.
Tire pressure will help your mileage but not the difference between calculated and MFD value. One gallon out of 8 is 12.5% or about 7.5 mpg. I track every tank on an Excel spreadsheet and compare actual to MFD. The average difference for the 30,000 miles is about 3% or 1.5 mpg at about 50 mpg.

 

After you inflate the tires to the desired pressure reset the tp sensors by pressing the button on the lower dash. Higher tp=higher mpg.

Things are different. The sounds can be unnerving. Chances are everything is fine so just relax & enjoy the car.

 

The last line ("pip") will start flashing when the tank is really low. There's no way to know exactly how much is left when that happens. It might be half a gallon or it might be fumes. It is prudent to buy gas before it gets that low. (Know that you must put in more than three gallons at a time or the tank computer may not notice that you have added any, and it may shut down the car anyway.)

 

2009 Florida First Tank

If you throw away the data up to the first time you filled the tank, what does it work out to?

I'm also in Florida (flat south) and just did my first fill since dealer "fill". I shouldn't have waited so long to fill up, I guess. My tank gauge indicated half empty and I put 5.94 gal in. That seems to match the 11.9 gal capacity figure.

The MFD indicated 46.8mpg but my calculated came out to 40.7 or 13% lower. I expected the MFD to be 2-3% off, so I'm guessing the tank wasn't really full.

I haven't decided if the first tank stats should be thrown out as "uncontrolled", or included since eventually the first stat will be "averaged out".

 

Hi Folks,
I've had my Prius for 3 months and was averaging mid-50's until recently.
I'm also having some issues with fill ups lately, since it's been cooler weather. I'm in Northern California and about 35-60 degrees. I've noticed that I'm only getting about 300-350 miles per fill up for the last 3 or 4 fill ups, averaging about 48 mpg. I'm also only filling up 6-8 gallons at a time even though my gauge is down to 1 or 2 "sticks".
I understand that the ICE is used for heat and there's a different mix of gas for winter, having read other posts about poor gas mileage in winter and this isn't a problem overall.I'm not worried about 48 mpg in winter, that's still so much better than my other car. I am concerned about only 6-8 gallons per fill up, going from empty to full.
I read that there's a "soft" gas bladder and that is just more confusing to me in trying to figure out what might, or might not, be a problem. is there any chance the bladder could think it's full but not be?
I was getting about 500 miles per fill up in warmer weather, at ~55 mpg.

Thanks,
Michelle aka TombQueen

 

The fillup issue is the bladder. When temps get down near Freezing it really shrinks.

The mileage issue is probably short trips. If the engine does not warm up during the trip, the car does not go into Hybrid mode, and glide (that is use the electric motor sans engine power).

Grill blocking is what we do here in the Great White (but not so far) North. It get then engine up to temp quicker. In your situation, you can probably grill block the engine half of the lower grill without going too high in temps. We do the grill blocking with pipe insulation, split apart and put in the grill work of the grill. Remember to pull the pipe insulation out if you get a 75 degree day. Or do a long up-hill run into the mountains, rather than day-to-day short commute trips.

 

Anyone have experience driving 350 continuous miles at 85mph at the higher tire pressures?

 

As temps drop below 60 mileage begins to suffer. Partial grill blocking can somewhat offset this.

Your fills are in line with mine and are roughly what I get at 2 pips. I've been going down to 1 pip and that gives as much as 8.9--but most often fills below 8 gallons. With the present tank I hope to try waiting until the flashing pip. (Wanted to on the last tank but a nasty storm was on its way.)

The gas bladder is frustratingly variable in capacity and shrinks in cold weather.

 

Hi Shawn, are you on vancouver?
One friend of mine tell me that he knows you

 

I just did my first reliable fillup (same pump as last time) and, while not as bad as your difference, my multi-function display read 51.5 mpg for 391 miles and the calculated came out to 46.8 mpg for a 10% "error".

Perhaps this bladder tank design makes the variance on "full tank" more than we expect. With the non-linearity of the gas guage and the variance on filling the tank, I am so not trusting anything the car tells me on the MFD.

I expected a difference between calculated and MFD, but my gut was saying that it would be the MFD that is wrong. (Wow, three tenses in one sentence. My English teacher would be proud...or something.) Until I get about 10 tanks "full", I won't know which "liar" to boast with. (Not that either is bad compared to the 20-21 of my last car.)

Tovli