Inconsistant MPG Data ('08 Prius display vs. Ultragauge)

[broski499]

Here is what is happening. The internal Prius MPG gauge will gave about 43.5 MPG last tank. My Ultraguage said that I was getting about the same 43.2. Yet when I filled up my calculations showed I only got 40.8 MPG.

The Prius also alerts me that my gas is low when the ultraguage still shows I have 3 gallons of gas left.

Has anyone experienced these issues before?

[Fat Charlie]

No real issues here. 43.5 is only 1.06% higher than 40.8. There are so many variables involved that having the gauges and the pump only 1% off from each other is pretty damn smooth.

The factory fill warning would tend to be conservative- it's not there so you can decide how far to take the tank, it's to give average people time to work a gas stop into their schedule.

[dirtydave]

^^^^ yeah 3 / 10 sounds like a good time to refuel.!!
And everything looks very close to each other soo uhh its fine..?
Do you use the same gas pump every time? Remember this is a passenger car it doesn't really matter if the gauges aren't 100%Close enough is good enough for me.


Maybe increase the engine size in UG to report a lower MPG number so you are suprized every time?

[broski499]

Thanks for all the info everyone. Just made a trip up the grape vine and my gauges were showing high 40s but when I manualy calculated I got 52. It's funny when you actually calculate the difference percentage wise it's a very small difference when 40 to 43 feels like a big difference.

Merry Christmas Everyone!

[oldtamiyaphile]

Originally Posted by Fat Charlie

No real issues here. 43.5 is only 1.06% higher than 40.8. There are so many variables involved that having the gauges and the pump only 1% off from each other is pretty damn smooth.

You mean 6% off. That's on the high side of the accepted margin of error for a throttled engine. I'd recalibrate the UG and ignore what the OE instruments say.

[broski499]

I thought that seemed high. I will definetly reset my ultra gauge and make sure all the custom settings are correct. I know I have the engine displacement set correctly. Not really sure what else I need to do.

[Fat Charlie]

I mean 1%. Call it 2% if you want.

40.8 mpg = 5.765 l/100km
43.5 mpg = 5.407 l/100km

40.8 x 1.066 = 43.49
Using "normal" numbers, the distances reported per gallon are close enough to 1.06% off to use that as a number.

5.407 x 1.066 = 5.763
Translating into metric and working backwards, the amount of gas used to travel 100 km are also close enough to 1.06% apart.

Those numbers are close enough that I'd call the UltraGauge calibrated and move on with my life.

[oldtamiyaphile]

Then you sir have failed maths class

A factor of 1.06 is not the same as 1.06%, it's actually 6%.

[wdb]

40.8 is 2.4 MPG or 6% lower than 43.2. That said, if calibrating a UG is anywhere near as hit/miss as calibrating a SG, I'd try tweaking it once or twice at most and then settle into using it as a relative measure.

We once went 50 miles in my wife's CRV with the low fuel warning light on. Drove her crazy.

[Fat Charlie]

Thank you!!!



That's why I posted, because tapping away on my phone made it look fine to me and I wanted someone else to look at what I was doing to confirm or deny the accuracy. Maybe now that Christmas is gone the kids, traffic and all sorts of extra events will calm down or go away and I can start to see straight again.

What really matters here is which gauge agrees more with the pump.