Guess Guage LOL

If the refill warning triggers consistently at the 8-gallon mark, there's nothing to calibrate. That's what it (the Iconic model gauge) is intended to do.

The catch is, some owners desire a much greater capacity/range than what that offers. And that's what contributes to the refill inconsistency.

It will always be off by some with the MPG displayed, since that is only a rough estimate based on measurement that includes rounding errors. A squirt of gas is precisely how much? And with 6,000 of them happening per minute, it's easy to see how the value could be off.
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How much mpg discrepancy do you get if you average over four or five tanks full?

 

I've found that the gauge is off slightly more in the winter (reads an ave of 51 when I'm getting 48) than in the summer (reads 59 when I'm getting 57). That's based on only 1 summer and 1 winter, though.

Today I filled up at 3 pips because I wanted to see if my recent P&G combined with the 45-55 degree weather has been having an effect.

My dash estimate from the computer was 59.9 - I actually got 65+ (5.21 gallons over 342 miles).

Bet next tank I get something like 47 again.

 

The amount you just put in usually does not equal the amount you burned since the last fill. That's why manual MPG calculation is unreliable. The MPG meter on the dash display is more accurate.

 

I agree, Richard. The bladder fill is not consistent, and as such you can't use the fill amount as what was actually burned over the last tank life.

 

Your conclusion is correct if you are talking about individual fuelings. However, over the long term (where fueling-to-fueling discrepancies average out), my experience shows that in my Prius, the MPG display on the MFD is optimistic by about 2.6%.

I have recorded the amount of fuel added and odometer reading each fueling over the 20,000 miles I have driven this car. I have also recorded the MPG display and reset it at each fueling. Using this method, I can calculate the volume of fuel the MFD indicates should have been used since the last fueling. The total volume of fuel added (according the the gas station's fuel pump) over the 20,000 miles was 414.29 gallons. Calculating the total volume of fuel predicted by the MFD over the same number of miles yields 403.86.

That difference, in my opinion, is pretty trivial. In addition, both calculations assume that the miles displayed by the odometer are accurate since that number is used in both calculations of MPG.

 

And the fuel pump is temperature correcting to 15C here, but the car is probably not doing this correction. So, I guess this time of year when the pump tells me it put 25 litres of 15C gas in the tank, where it then contracts a bit due to the lower temperatures,.... then if the car thinks it burns 25 litres before the next fillup, it's actually a bit more empty when I fill up that second time.... and will report an optimistic number?

 

Your suggestion about the lack of temperature compensation is interesting...

Several people have the reported that the MPG display appears to be one or two percent optimistic. Other possible/partial explanations for the discrepancy include:
- Toyota deliberately made it a little optimistic to make us feel better. But then why only two percent?
- Evaporative losses from the tank: some of what you put into the tank never gets to the engine. But the vapor recovery system should prevent most of that.
- Some gas pumps overstate the amount delivered. There is an economic incentive for this and it does happen. How often/how much is hard to judge.

Yep, I should have qualified that as "manual calculations for a single fill are unreliable".

 

That's common and as already suggested the MFD is more accurate on an individual tank basis. Most people find that in the long run the MFD is ~2% higher than manually calculated MPG. Do pay attention to the PIPs and consider refilling at 3 PIPS