Gas milage

Almost everyone (it happens to my 2006 regularly). Search for "guess gauge" and you'll find a plethora of answers.

 

:frusty: Arrrggggg! This has to be the most often asked question. We should make the answer part of the signing-up process.

Okay, deep breath; I'm back in control. The N.A. models of the Prius have a bladder in the fuel tank. The bladder does good things for the environment, but it causes the usable volume of the fuel tank to vary from fill to fill. The result of this is that your manual calculations are unreliable - unreliable to the point of being useless. The only way to get useful manual mileage calculations is to do them over a very large number of tanks. This spreads out the error term, reducing it to a manageable level. How many tanks? It depends on how much accuracy you need. As a starter I would say you need at least ten tanks.

Another thing to keep in mind is the accuracy of fuel pumps. Your manual calculations are based on the volume reported by the gas station's pumps. The pumps are fairly good, but not perfect, and the error in gas station pumps tends to be one sided.

Tom

 

I'm with Tom!!! I do not want to be annoyed with new members over something so silly as repeated posts but this does get annoying. Can we help them out by posting it as a bold sticky in this forum please?

 

For the benefit of the OP, here are my tank-to-tank fillups and differences between the MFD and calculated MPG figures since October. The "weighted difference" is weighted based on miles per tank, and the cumulative difference averages that column.

 

To the OP one gallon out of eight is 12.5%. So, if you don't top-off to the exact same point it is easy to have an error as high as that.

In ten months my total difference between theoretical and actual is 3%.