Even when pumping at the same station and same pump, variations will occur. Pressure is never consistent, and depends on how many pumps are running, etc. Unless you're filling it right up to the opening each time, (not recommended), assume that variations have taken place. This is why you should average over many tanks.
For some reason the Prius mileage as computed on the dashboard on my car acted up. With 7000 km on the odometer, all of a sudden it showed a lifetime mpg much worse than the day before (5.6l/100 km or 42mpg) (and much worse than the manual calculation). About a day later the lifetime mileage went back to where it should be (in my case 4.8l/100 km (49 mpg US) Strange, and not sure why it happened--I mean I can see the trip a or trip b mpg figures diverging from the long term average, but there is no way a car with 7000 km can have a mileage figure of 42 mpg one day and 49 the next (with hardly any driving in those few days). And yes, I triple checked and wife checked also..it was the lifetime average.
If you're thinking there's a corellation between speedometer discrepancy and mpg error, I'd disagree. The first is mandated, the second is Toyota trying to sugar-coat, imho.
If you are watching fuel economy and press the History button on screen, and press reset to make last tank (or whatever value is current) into a column on the graph, then it will reset your lifetime MPG that shows next to the lifetiem odometer reading. Could this explaing your situation?
The one thing you can depend on is the odometer reading. Everything else is risky. Personally I reset the trip meter at each fillup, but use it just for feedback. I record the odometer reading at each fillup, keeps me safe from inadvertant resets.
Bingo! That is exactly what I did and what happened. I had presumed the "lifetime average" was for the entire life of the car and could not be reset, but now after your posting and checking the manual, I realize it can be reset. I remember drving the car home and pressing that button "just to see what it would do". Oh well, it is exactly where it was before and I use fuelly as well. Thanks for the posting.
My bride reports on her last fill us the screen showed 43.2 and her manual calculation was 42.7. Pretty close. She is not real picky about trying to fill up to the same point each time. Of course, she only has about 1600 miles on it, including two 500 mile round trips mostly interstate at 75 to 78 mph. And I just went out and checked her air pressure. The dealer sent her home with 30 psi in all tires. Had she been running 42/40, I would expect the mpg to be somewhat higher. I have now brought tires up to 42/40. We'll see what the impact will be on the mpg.
I recently replaced the tires on my Prius v with Yokohama Envigors and was surprised to see an instant drop in MPG. However, I soon realized that the drop was actually due to much more accurate readings reported my the on-board computer. Like many others here, I had reported a 5% discrepancy in reported speed and distance which led to a similar overstatement of gas mileage. Now, the speed and distance are very accurate which should also lead to accurate MPG (although I have not actually checked this). Even though the tires are the same stated size as the originals, there must be enough variation between brands to cause this rather significant error.
