BIG swing in mpg's on consecutive tanks - ???

Welcome to one of the great anomalies of the Prius.

Also, you get an award for asking perhaps the most common question posed by the new owner.

The short answer is that you'll see this because of the rubber bladder than lines the gas tank. The bladder will change sizes and you will not get the same amount into the tank each time. The MFD (the multifunction display -- the screen) provides a much better indication of your actual mileage.

Here are some threads to read to get fuller explanations:

1. New Owner? Want MPG help? Read this first.
2. Mileage vs pump calculating.
3. Why is the milage on the screen always different than real milage?

Hope this helps!

 

Where I live, there is no choice about ethanol in the fuel ... it's in by law, so that is not an issue for me.

Over the past 40 + years, I have learned that no 2 pumps shut off automatically the same. The cut off is controlled by an air pressure sensor in the nozzle ... and even with the same nozzle ... the cut off can change during the day as temperature changes.

I ALWAYS set the nozzle at its slowest speed ... slow gives me a more reliable fill than fast ... which can trigger the cut off sensor too soon.

I never worry about miles per tank ... there is no such measure ... for 4 decades I have measured miles per gallon ... which is a standard of measure that NEVER changes.

One thing I have observed with my Prius ... because it is capable of high mileage numbers, is that mpg seems to be very much tied to temperature ... and in turn, the temperature of the battery pack. When my car sets out in the sun on a 100+ degree day ... and gets really hot ... my mileage drops to about 45 mpg. On a cool run, after being garaged overnight at 75 degrees, my mileage is more like 55 mpg. I have read as high as 58 mpg driving to work at the Dallas/Ft Worth international airport ... driving in the Dallas metro ... on such cooler days.

My lifetime mileage is just above 50 mpg ... over 8000 miles ... which includes Dallas metro in town and 70-80 mph trips on the interstate.

 

I hate to burst all the my fellow Prius owner's bubble out there but this phenomenon isn't exclusive to the Prius and the "fuel bladder". That is unless my 1999 Corolla LE and 2002 Corolla CE also contained "bladders".

I drive a ton for business and have done so since 1996. My 2006 Prius has over 126,000 miles already and both the Corolla's had over 100,000 when I got rid of them. Many of these drives are two and from the same customer on the same roads here in the south.

As a result I have had the oppurtunity to make almost the same exact trip, minus minor "pit stops" for bathroom/food/etc..., on more than one occasion in all 3 vehicles on consecutive tanks of gas.

I too have noticed the "1 high MPG" then "1 low MPG" tank on all three vehicles. To be high/low I would consider a difference of greater than 6 MPG. The first response will be well you filled up at different gas stations with different pumps. Nope, wrong. Not always but again more than once on all 3 vehicles on more than one occasion I filled up at the EXACT same station often with the EXACT same pump, cars are creatures of habit, LOL.

It drove me NUTS!!!!! Finally I decided to stop trying to figure it out, which as an engineer admits defeat, LOL. Honestly though I have spoken with dealerships, other engineers, NASCAR engine builders, etc... no one has a "good" answer for it.

They have given the obvious "road conditions", "head winds", "tail winds", "driving habits", "drafting" (yeah that one came from certain team member here in Mooresville), "gas differences", "air filter", "dirty EFI", "tire pressure", etc...

Yeah yeah yeah all those things could have a small affect on MPG but not the large difference I was seeing. And obviously not consistently from 1 tank to the next as it was and very repeatable. So, I gave up and would recommend you do the same for sanities sake and just love that fact that your Prius gets such good gas mileage

It is almost as if the cars had a "reserve", for those of you that ride off road motorcycles. And the car was like "Ok, let's screw with him and use the reserve tank this trip but not the next." See the depths to which I have contemplated this issue, it annoys me. On that note I must end this before the men in the white coats come.

Just my rambling $0.02.

 

You make a good point. It's hard to refuel any tank to exactly the same level, unless you can inspect the level while filling. Fuel pumps click off at different points, parking lots are uneven, and fill rates vary.

The Prius fuel bladder just adds one more large variable into the mix. The problem isn't unique to the Prius, just exaggerated.

Tom

 

I would think that the Prius' lower consumption and smaller tank would both naturally cause more variation as well. +/- 1 mpg on 25mpg will right off the bat give you +/- 2mpg on 50mpg for a fixed filling error in gallons. The same fixed filling error is also a more significant part of the whole tank for a smaller tank.

Rob

 

So far my tanks have been (off the top of my head, should be close to accurate):

52.7
51.0
48.5
53.0


First tank was from the dealer. Next two tanks from Hess. Current tank (still on the rise) from BP or Shell. I forget. All I know is I didn't see any ethanol signs.ß