Filled up at different station/pump - big difference

[mwr]

Today I filled up at a different station (and pump, of course) than all of my prior fillups which had been at the same pump. Before, the difference between the displayed mpg and actual mpg was about 2 mpg (actuall was lower). But this time, the displayed was 46.8 mpg and the actual was 42.5 mpg, a difference of 4.3 mpg. I assume this pump allowed fiilling the tank "higher"/more full than the proor pump. Agree? Over a longer time and miles period, this will average out but it's bit of a rude surprise at the time.

 

If I can fill up next time at the prior pump, if my theory is correct then I should regain that "lost" mpg :-)

Edited January 22, 2015 by mwr

[corncobs]

Yes this happens quite frequently. My regular gas station I'm usually fill slightly over what the car tells me it has used.

 

Yesterday while on trip I fill up and it clicked at 1 gal over the car display 9.1 gal used vs. 10.2 gal filled. So instead of 39.x MPG the real # calculates to 35 MPG.

 

Like you said the next time it might click at or below the cars gallons used calculation which evens out the average.

[hybridbear]

Our entire road trip we put in slightly more than the car showed at every gas station. When I filled up at home I couldn't get as much in as the car showed. The pump shut off more than 3/4 of a gallon early and I had a hard time adding more gas.

[lolder]

There is a big air space in these fuel tanks. The angle of the car, the rate of pumping affect when the auto shut-off occurs. Fill-up calculations are more accurate at the same pump with the same fill rate and auto shut-off and no additional clicks. They are not as precise as the cars computer. It has been pretty well established that the car's computer reads about 4% high. The odometer in the 2010-12 also reads 2% low for a net of 2% too high a mileage figure. Many fill-ups should be used to confirm the error and once that is done you can rely on the car calculations.

 

Accuracy is how close to the real number a measurement is. Precision is how finely you measure it. The car computers are very precise but have a fair amount of bias inaccuracy.

[mwr]

Filled up at the original station and pump today and, as predicted, I gained back the mpg that I lost by filling at another pump last time that allowed more fuel to enter. This time, 500.3 miles, 10.785 gallons, car displayed 46.2 mpg, actual was 46.4 mpg. Normally, the actual is about 2 mpg less than the displayed. Last time actual was 4.3 mpg less than displayed, but this time the actual was 0,2 higher than displayed, effectively cancelling out the lower mpg last time.

[Texasota]

Filled up at the original station and pump today and, as predicted, I gained back the mpg that I lost by filling at another pump last time that allowed more fuel to enter. This time, 500.3 miles, 10.785 gallons, car displayed 46.2 mpg, actual was 46.4 mpg. Normally, the actual is about 2 mpg less than the displayed. Last time actual was 4.3 mpg less than displayed, but this time the actual was 0,2 higher than displayed, effectively cancelling out the lower mpg last time.

I frequently experience the exact same thing.

[md13ffhguy]

That's why when you say "actual" mpg, it's not really actual, but manually calculated based on the actual amount of gas purchased. Over time, I guess it averages out, and gets closer to actual. That's why our Fuelly graph lines are up and down from one fill-up to the next, rather than a nice, smooth trend-line.