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

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?

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.

^^^^ 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?

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!

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.

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.

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.

Then you sir have failed maths class :rolleyes:

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

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. :)

Thank you!!!

http://www.quickmeme.com/img/00/000b...710f05564b.jpg

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.

Don't be so sure that your fill up calculations are more accurate than the computer. The Gen2 Prius (2004-2009) in North America has a rubber bladder in the gas tank that expands and contracts to the amount of fuel in the tank. This can cause inconsistencies when fueling up, especially when the outside temperature is different than the last fuel up because the rubber can't stretch as much when its cold. I've had several pairs of tanks where the first one was quite a bit higher or lower than the computer and then the next tank was the opposite. Because of the bladder, you'll probably never have the gas tank completely full. I try to fill up when I get to the last 2 bars.

The computer is usually about 2MPG higher than actual. I personally think its the most accurate reading for a single tank in a Gen2 Prius.