How accurate is the mpg reading?

I think the mpg reading can be misleading to a lot of new Prius owners like me. Everytime I fill up my tank, I just calculate my mpg. Tonight, I filled up with three bars left on the gas gauge for 6.779 gallons. That makes 43.51 mpg from my last tank. Previously, I had 48 mpg....oh well, I'll blame it to this winter season. At any rate, I am still happy with the mpg that I am getting.

 

Firecube2426, The (MFD)dispay MPG is very accurate. By averaging many tanks and comparing hand MPG vs. display MPG, many drivers have reported that the actual (hand calculated) MPG is ~0.5-1 mpg lower than the car displays. Using my numbers over 14000 miles my actual lifetime MPG is .36mpg lower than the MFD lifetime.

As to the gas gauge the only reason people say 1 gallon of gas per pip is because there is ~10 usable gallons and 10 pips. In reality you will most likely show a full tank for the first 75-150 miles of driving, and then every 30-50 miles another pip dissapears(as you have noticed). Driving in this Michigan winter weather like you, I am getting 49 MPG for a 12 mile commute. My typical fill-ups are 350-420 miles for 7-8 gallons. Good advice would be to block the grill (~4MPG boost for me) and keep the heat off for the first 5 minutes; after that I use medium fan speed and it heats the car right up. Look forward to the summer when we'll see 52-65 MPG.

 

The gas gauge is both nonlinear and, because of the fuel tank bladder, variable. Don't try to estimate how far you should be able to drive before you need gas. People who do this and ignore the flashing of the final pip on the gas gauge eventually run out of gas and feel very silly.

You can't assume that gas pumps are more accurate than the MFD. The law allows a few percent of error, and gas station owners have a financial incentive to have their pumps deliver less gas than is displayed on the pump. And that makes manually calculated MPG less than the true MPG.

 

This is why we call it "the guess gauge". In my experience the MFD is usually very close to my calculations and almost always high. However, sometimes there can be a 10% difference. To improve your fuel economy block the grill, increase your tire pressure, and install the EBH.

 

JimN, you seem to highly recommend blocking the grill. Can you share some photos of a 'blocked grill' for future projects?

 

I found my first tank MFD to calculated ratio very close (7.2 to 7.27) but my second tank was VERY different (5.0 to 5.65) (all numbers are L/100 KMs). Obviously, I will continue to pay attention and record my consumption, but I was wondering if there is a known cause of significant (>10%) differences?

Thanks.

 

Answered my own question by checking out the spreadsheet one of the other users posted about his consumption. Overall and over time, MFD is very accurate, but any one individual filling can be quite off...

 

The MFD is always accurate. Individual fillings vary. This may be what you said, but your statement was ambiguous.

Tom