What would cause my Prius to do this?

big_scott_larock

I usually get about 41 to 45 mpg. However, this last time that I filled up, I only got 35mpg. But, the car said that I got 44 mpg. What could cause this? Normally I would say its a bad O2 sensor, but the Prius doesn't have one.

JerryJ

Sure it does but that's not likely why. The Prius has a bladder which can have a different capacity depending on things like temperature and the gas pump's state of repair. You can't use the pump calculation for a single tank on any vehicle that gets more than 40 mpg because it only takes a few ounces of difference in the fill level to make a material difference in the calculation. This goes double for a bladder equipped car because you can easily have over a gallon difference in fuel capacity. The pump calculation can approach accuracy if you average 10 or 20 fills together.































Here's the data from my 2004 Prius. Note that there are almost 30 fills out of 223 fills--110,000 miles) that had over a gallon difference in the fill level. What this shows is that the display calculation is good enough for anything outside the laboratory.















































Difference between pump calculation and MFD:































As of June 28, 2010 fill















"All" is all tanks"Some" excludes the tanks that have over one gallon difference.































______________All_____Some















Total_diff__72.73____38.51















Ave_diff_____0.33_____0.20















Median_diff__0.35_____0.27















Max_diff_____1.94_____0.89















%_over_______3.6%_____1.9%















Count________223______194















Total gas purchased:__2016.3 U.S. gallons















Total gas calculated:_1943.6 U.S. gallons