What accounts for this?

If you use ethanol (89) vs non ethanol (87) you will see variations in MPG. The biggest change is I see in MPG is the wind; head winds, crosswinds, etc.

 

Don't forget with the bladder gas tank it is difficult to know exactly how much gas was consummed since it can trip off the gas pump prematurely and throw off your gallons of gas value.

 

Fuel formulations and tank bladder causing filling variations will be the biggest culprits... aside from the nut behind the steering wheel.

 

I suspect the pump & if you were in NJ, the attendant. Once it clicks off do you continue to top off? I don't top off & had very few wild swings.

 

Variation from the fuel bladder.

Tom

 

The bladder stiffness could have changed with time and temperature, but more likely it is the gas pump. Tank venting issues can also influence fill levels.

Tom

 

I'll bet a nickel that your MFD values have remained consistent, in which case it's variability in the size of a fill (bladder stiffness, wacky nozzle shutoff, a venting problem).

 

Hobba,

As expected on a Prius forum you will here everythign about the gas tank bladder, pump differences, shutoff variations, time, temperature, tire air pressure, etc.. etc...

Don't get me wrong these are all very valid explanations BUT and this is HUGE. I have had the EXACT same problem you have had not only on my Prius but also on 2 different Corollas, a 4-Runner and my current Tacoma. Yeah I buy a lot of Yota's, LOL.

Anyway it isn't always one high then one low sometimes maybe most of the time they will be consistent from tank to tank but then they seem to get in a rut and just go one high then one low then high then low. It really is annoying.

http://priuschat.com/forums/gen-ii-prius-fuel-economy/53085-big-swing-in-mpgs-consecutive-tanks.html

That is the link to another thread that discusses it as well. My post in there says "I gave up." If you come across an answer please let me know.

Happy Driving,
Chris

 

It's not a mystery. The variation comes from variation in tank fills. Figure out how to be consistent when filling your tank and you will eliminate the variation.

Tom

 

Hobba, are you saying that the MFD value *for an entire tankfull* (that is, the value on the display immediately before you fill up again) is also wildly different from tank to tank?

 

You pretty much have to accept the tank-to-tank variability being larger. The combination of bladder and non-venting system makes the fill process more sensitive than any other vehicle I've ever used.

What can really screw you up when filling is having half a dozen other people filling and topping off their cars at the same time. When their nozzles shut off (at full flow rate) it sends a pulse down the line that will sometimes cause mine to shut off. Even if it doesn't shut off I can feel the pulse in the trigger. This can result in some guess work on a fill after mine kicks out way early.

And sometimes, the tank just doesn't want to fill.

 

No, or not necessarily. If you pump 10 gallons to full and had driven 350 miles, then it would calculate at 35 mpg. However, if you pumped 10 gallons to full, then drove 350 miles you wouldn't know the mileage on the tank until your fill at the end of the 350. The fill tells you the calculated mileage for the previous tank of gas. And with the typical variability in the tank volume/venting issues that next fill might be 10 gallons...or 9 gallons...or even 8 gallons. If it is really giving you fits you might only get in few gallons leaving you with a calculated mileage that is twice that of the MFD (personal experience.)

With other vehicles there is more predictable fill variation. In my experience it coincides with the inclination of the tank during fill. I chose pumps that allowed me to park level or gently sloping away from the pump--never toward the pump as this resulted in some lost tank capacity.

The relatively small size of the Prius tank probably also contributes. Being short a gallon in a 25 gallon tank is not a big effect, but being short a gallon in a 10 gallon (effective) tank has a quite noticeable impact on range. And although the tank size is smaller, the surface area and headspace issues don't decline proportionately. Plus the tank still must be able to vent the same volumetric vapor flowrate as a 25 gallon tank--because the station pump doesn't change its flow capacity based on what size tank a vehicle has.

 

If the MFD mpg is swinging wildly AND pretty much matches the spread sheet, then it CANNOT be anything to do with the fill technique. I have no idea what could cause it but I would worry a lot that it was a sympton of some type of imminent serious failure.

 

Mine too dropped to 38MPG last tank think its the heat here has been 100 here in Ocala....

 

Thank you for not topping off. Topping off only increases hydrocarbon pollution and may damage the fuel system.

But Yes, 26 MPG shown on the display, if it is the cumulative result of burning an entire tank, is worrisome. (Getting that during a single two-minute trip is not worrisome.) Assuming that you have enough air the tires and are not doing something goofy with the car I'd take it to a dealer for diagnosis.

 

If you are getting 26 mpg in your Prius, you must have a body dragging underneath. Perhaps you have brake drag - any heating of the brakes? Something is definitely wrong with a mileage figure like that, unless all of your driving is very short trips.

Tom