I've driven 170 miles, averaged 49mpg, and I've used half a tank

since you have a 2010 prius, this shouldn't be caused by the gas bladder...

Anyone know if the 2010 gas tank has the inclination calibration that could be done on it?

 

Yeah? So?

All this seems fairly close to normal gas gauge behavior, Prius and otherwise.

The distance to empty reading doesn't show how many miles until the tank will take 11.9 gallons. More commonly, 'zero' miles corresponds to 9-something gallons. While many of us have driven well past 'zero', beware that at least one GenIII has apparently run dry very shortly after zero.

Throw in the normal variation in gauges, fuel sloshing around in the tank and causing erratic readings, and different gas pump shutoff levels, and your reports seem within normal bounds.

 

We have found that the fuel gauge on our 2010 Prius reads full for the first 2.5 gallons and we still have about 2.5 gallons of useable fuel at one bar. Together, these represent nearly half a tank of gas that can appear to be missing if your fill is incomplete and you get gas a one bar.

 

Did you use A/C during those 170 miles? I've found that my in-dash mpg meter does not "tick down" when the A/C is on. However, the gas guage and distance to empty will on your trip meter. If you track your consumption with a fuel log you will notice a larger than normal gap in your in-dash mpg and fill mpg...especially when A/C is used heavily during a tank.

Also, consider your filling method. 1 click, 2 clicks, top off? Did you fill up at the same gas station and tank you always do...sometimes pumps will hit 1 click sooner than other pumps.

Could be many factors but I doubt it's anything mechanical.

 

When I last filled up (a few days ago), the last pip started flashing and I got the audio beep about 1 mile from the gas station (~ 20 mile to zero on the display). When I filled up, I put in 9.5 gallons. (9.2 gallons after the initial shut off, and about 0.3 gallons to the first click/shut off). Meaning that there was about 2.4 gallons left in the tank.

Typically, for my car, if the pip starts flashing, I assume I have ~ 2 gallons of gas left in the tank. I'm assuming it is by design that the pip flashes with that much gas left in the tank so that we "smart" drivers don't run out of gas. Cars have been designed like that for decades (Tank shows "E" but there is a gallon or two of gasoline left in the tank so you could get to a fuel up station).

It has been discussed on these boards, by people FAR more informed and intelligent than myself about the difficulties in accurately measuring fuel consumption on the fly. There are many engineers and such who love accurate and precise data and would love an accurate and precise "Miles to Zero" reading on their tanks. But it isn't a trivial task to program and, frankly, it isn't that important (not like, say, in an aircraft, for example).

In the end, you likely will know that you have ~ 1.5 - 2 gallons of gas left, which for a prius, is something akin to 70 - 100 miles to easily find a gas station and refill.

 

My miles to zero on the trip meter reference was not meant to say it should be used to accurately measure fuel consumption.

It was intended to show the effects of the use of A/C if the OP used it during the past 170 miles. It is a visual representation of how quickly A/C uses up fuel compared to monitoring the same guage with A/C off.

With A/C "on" the miles to empty trip meter ticks down much more quickly than with A/C "off." And while you would think that would effect mpg...it does not on the Prius in-dash mpg guage. It just catches up with you when you fill the tank and calculate consumption...and leaves some wondering why the fill mpg totals and in-dash mpg totals are off more than normal.

 

How many miles during the 170 did you use the A/C?

 

As stated above, the guage isn't anywhere close to linear.

 

You are at "one bar above the halfway point with the gas gauge", i.e. 6 bars left, or 4 bars burned. On mine, each bar represents roughly one gallon, maybe slightly less. When averaging 55-60 MPG on the display (which has varied 5% to 9% higher than reality this year), I'm reaching 6 bars just barely past 200 miles after a refill. Simple proportions suggest that with your lower MPGs, you would reach 6 bars just past 163-178 miles.

Add in any pump shutoff variation, any slope to the parking pad at your gas pump causing an incomplete fill, car-to-car differences, bar-to-bar differences, and any sloshing around in the tank from hills, corners, and stop-and-go (ScanGauge X-Gauge FLV display reveals a lot of slosh that is mostly, but not completely, filtered out by the car's display), and what you report becomes not at all surprising.

All these things (except the FLV display) applied to my pre-hybrids as well.

Until you can show multiple consecutive tanks of fuel with low calculated MPG, there is no cause for action.

 

How many miles does it claim until empty?

If your constant MPG is showing 49MPG your total miles on a tank should be 490 miles. Your miles before empty should be 490 – 170 = 320 (approximately). A half tank at 49MPG should show around 245 miles.

 

Without knowing how much gas was in the tank to start with it would be impossible to know what a half tank at 49mpg is. It is only an estimate and could be off by as much as a gallon (or 49 miles).

Also, running the A/C for any portion of the 170 miles driven will reduce the number of pips remaining and the miles to empty...but not necessarily in the in-dash mpg. This will throw off the calculated mpg...alot depending on for how many miles A/C was used.

 

He started with a full tank. You have to assume it is close to 11.9 gallons.

I have found this not to be true. If I run the AC a lot it is directly effects the constant MPG.

 

I verified today that A/C directly effects the calculated Const MPG on the display.

 

I know this question is in the Gen III slot ... it wouldn't be the 1st time someone asked a question in the wrong area. That said, what year do you have. The Prius fuel gauge, prior to the 2010 model are frequently call a "guess gauge" by many. Put your year on your avatar stat's and folks won't have to guess about that too.


.

 

Looked up some of his older posts, he has a 2010 prius, not a gen 2.

 

I have found that running the heat effects the const MPG more then A/C.

How many miles did it show remaining on the tank after 170 miles?

 

I tracked a few tanks last year. Obsessively looking at the gauge every few miles. It averaged out to the following for me.

Gauge - miles driven - delta
10 - 0(@fill)
9 - 45 - 45
8 - 94 - 49
7 - 150 - 56
6 - 176 - 26
5 - 234 - 58
4 - 287 - 53
3 - 337 - 50
2 - 390 - 53
1 - 428 - 38
blink - 455 - 27

I note that sometimes the gauge will drop a bar, and then later when I restart the car, the bar reappears. Even over level ground. I guess it may have something to do with a pressurised tank. Sometimes it takes 10 more miles for the same bar to drop out again.

In many cases, I note that I can easily go 100 miles extra after the beep/blinking begins. And I have never run out of gas (knock-on-wood). Although usually fill up between 50-75 miles after the beep. I figure worst case I have had is 47mpg x 11.9gals = 559 mi. So I try to never go past 550 (unless the tank mpg shows above 50mpg ;-).

Do I need a life?
rich