Where's my MPG going?

[bugmenot]

I fill my tank till the pump automatically shuts off,But I do realize some pumps wont shut off automatically,or will shut off prematurely.
I know I could probably pump another gallon,if I was willing.
This could possibly affect your mileage up to about 5 mpg if my math is correct.

[b_hickman11]

Originally Posted by bugmenot

I fill my tank till the pump automatically shuts off,But I do realize some pumps wont shut off automatically,or will shut off prematurely.
I know I could probably pump another gallon,if I was willing.
This could possibly affect your mileage up to about 5 mpg if my math is correct.

This has nothing to do with it because you will still be dividing the number of miles driven by the number of gallons pumped. You don't divide it by the size of the tank.

[WeeYari]

Originally Posted by b_hickman11

This has nothing to do with it because you will still be dividing the number of miles driven by the number of gallons pumped. You don't divide it by the size of the tank.

Wrong.

Since you are calculating based on distance travelled and the amount of fuel needed to refill, your calculations can be scewed since the "full" level can differ from pump to pump.

[b_hickman11]

Originally Posted by WeeYari

Wrong.

Since you are calculating based on distance travelled and the amount of fuel needed to refill, your calculations can be scewed since the "full" level can differ from pump to pump.

You divide the number of miles traveled by the number of gallons displayed on the pump. If you only pump 8 gallons into the car then thats what you will divide by. If you only pump 2 gallons into the car then thats what you will divide by. It doesnt matter how big a gas tank is, or when the pump shuts off.

For example, if I have driven 385 miles, and I go to a station to refill, and I pump 10.5 gallons, then my mpg is 36.67. You don't divide by 11.1 or the size of the tank.

Another example, if I have driven 75 miles since my last fill up and I pump 2 gallons in the car to fill it up, then my mpg would be 37.5.

So therefore if YOU fill the car up and you have cleared your trip meter after the last fill up, you should get a fairly accurate mpg reading.

[WeeYari]

Originally Posted by b_hickman11

For example, if I have driven 385 miles, and I go to a station to refill, and I pump 10.5 gallons, then my mpg is 36.67. You don't divide by 11.1 or the size of the tank.

So therefore if YOU fill the car up and you have cleared your trip meter after the last fill up, you should get a fairly accurate mpg reading.

Obviously you don't divide by 11.1

But you get different mpg calulations if one pump shuts of at 10.5 gallons, while another pump could shut off at 10.6

Regardless, for basic monitoring calculations would be fairly accurate, but pumps will contribute to either apparent gains or losses in mpg. We really see it with L/100km monitoring. Its easy for pumps to differ 1/2 litre from each other, and that 1/2 litre is a significant % of consumption.

[b_hickman11]

Originally Posted by WeeYari

Obviously you don't divide by 11.1

But you get different mpg calulations if one pump shuts of at 10.5 gallons, while another pump could shut off at 10.6

Regardless, for basic monitoring calculations would be fairly accurate, but pumps will contribute to either apparent gains or losses in mpg. We really see it with L/100km monitoring. Its easy for pumps to differ 1/2 litre from each other, and that 1/2 litre is a significant % of consumption.

Thats why I said YOU fill up the car not the pump. If YOU fill up the car then it will be accurate. I took drivers ed. at AAA and they taught us to fill up the car after the automatic noozle shuts off inorder to determine mpg. A lot of my local pumps will shut off and then I can still pump about 2 gallons into the car. It's common sense!