I've read threads here about issues with the fuel sensor, but I don't know if those cover my problem exactly. Basically, I want to know why, on one of my trips (380 or so miles), the computerized mpg gave a figure of 54.8 mpg while my own calculation (miles driven divided by the gallons I added to the tank) gave me 52.2 mpg. Another trip showed the HSI mpg at 56.3 while my own calculation gave 51.1 mpg. Someone please tell me what's going on here. I'm not that worried since I'm still getting good numbers, but it seems very confusing, and the differences between the two numbers can be fairly significant.
There is a good discussion on this topic in the following thread: http://priuschat.com/forums/gen-iii...-mpg-please-post-your-results.html#post899099
The difference is: 1. You aren't filling up the tank to the same level each time. Even it you let it quit when the nossle cuts off, its a different fill level. Also different nossles cut off at different times. They aren't all set the same. The -only- way you can know it's filled to the same level is if you fill it -completely-. This is not recommended. It can damage the vapour recovery system. 2. The car calculates the fuel used by the injector firing. This is pretty accurate, but not exact. Your errors indicate a 4-10% accuracy. That's very good! 3. Fuel volume changes with temperature. If the car calculates, say 10,000 injector firings, the actual amount of fuel can be different at different temperatures. The car is unlikely to account for that, as it's not intended to be a lab. quality fuel measurement device. You couldn't afford that! 1 MPG difference at around 50 MPG is 2%. You're fooling yourself expecting that kind of accuracy. Most mileage indicators are +or- about 10%. Add to that the filling inconsistency, and you get the kind of numbers you are seeing.
David, there's a bigger problem than that on The GenIII mpg display as noted by many owners. 4-10% accuracy is pretty awful, especially compared to the GenII which in my case is off by just under 1% so far. If the display was 100% accurate with the metered fuel, one would still expect a small negative departure though for actual mileage (e.g. small loss a the pump nozzle, any making it's way as vapor from the vent recovery system, and perhaps others.)
There's already been a lot of discussion on the inaccuracy of the displayed mileage in this thread: http://priuschat.com/forums/gen-iii...y/66149-2010-toyota-prius-fcd-inaccuracy.html
