I have noticed the same I almost made it to 140miles on the first blip. For others I when I have reached the last blip blinking, I fill my car up and only put 10gals in it. You at least have 1.9 gals left.

 

I keep reading about the gas gauge and its possible inaccuracies, so am wondering: Should I, in contributing to the mpg database, not be using my actual calculations? Because they're not as good as the onboard computer (which claims I'm getting several more mpg than I actually seem to)?

I'd kind of assumed up to now that actual fuel consumption would be the gold standard, but is that true of a bladder tank? And how does the computer figure mpg?

 

Oh, another! Welcome to the MPG minus 5 discussion!

When I first got my Prius 2007, it was frustrating that the manual gas amount I paid for at the pump divided into the odometer mileage is always 5 MPG lower than the computer! The computer has been exaggerating!!!

But after a year, the difference is shrinking. Now my manual calculation is about 1 MPG lower than Computer (reasonable).

In comparison, it should be mandatory that all users should use manual. (Again I'm sure many of us will re-start the "how do you know xxx is accurate" debate.)

 

The computer uses injector on times and rate to calculate how much fuel was injected into the engine, then divides that total by the distance traveled.

Pearl is pretty close on this. Last tank (filled up yesterday) was right on compared to my calculations (amount of fuel added and distance traveled).

You will note that it sometimes takes 20 sec. or so to finalize at a MPG number (like when you're waiting at a light the tank MPG number will change, even though the engine is not running).

The fuel gauge is a separate device. It's "stand alone", that is, doesn't rely on the other calculations. The fuel gauge won't reset if you only put a very small amount of fuel in the car. It takes 2-3 gallons to trip it. Some claim there is a sensor in the filler neck, but I think the gauge is actually operated by a microprocessor, and it watches for changes in the float height and uses that input and some logic to assign "pips" to the display.

Note also that when the engine stops there is still about a gallon in the fuel tank, in the depression where the pickup and fuel pump is, along with a depression below to collect "junk". Note also that fuel pumps don't like to run without being immersed in fuel (for cooling). It -might- be possible to overheat the pump just before you completely run out (the car shuts down the pump if the engine stops).

 

Sorry if I hit a sore point. I'd driven a rental Prius 1,800 miles this summer and had seen the opposite happen a couple of times -- gas mileage actually seemed better than the computer indicated -- so I was prepared for there to be some discrepancy, though not necessarily in the computer's favor.

Have only gone two tanks, though, and I'm already getting mileage twice as good as I got in my Jetta, so I'm pretty pleased. Just not quite as smug as the car itself seems to be...

 

Oh, and a cursory look through the Prius database suggests the higher MPGs for cars like mine that might not be broken in yet seem more likely to be associated with manual calculations. So much for bragging rights, I guess.

 

dave
thanks for post what prius was that ???

 

I know exactly where you are coming from on this. We have run out twice, both times after the warning ding we only made it a couple of miles. Once we made it to a station, the other time we did not. Now we always fill up when it gets down to 2 bars left. But here's the weird part, I can only put like 7 gallons in the tank and it's full again. I would think that I'm getting dangerously low considering how after that final bar I am flat empty. But it only holds like 7 gallons! I would think it would hold 10 or 11!

This makes no sense at all and every time I take my car in for service I always mention how the gas tank and gage are way off. They say no one has ever had this problem which I find hard to beleive. I want them to check it out but they don't think I have a problem.

It's like the tank don't recognize that there is 12 gallons in there, only like 8 or 9!! Am I crazy??

 

I once read that the Prius "gas gauge" is realy a "guess gauge"

 

One thing to watch out for is the fact that temperature has quite an effect on the physical density of gasoline. Remember the "heat expands, cold contracts (shrinks)" thing from Jr. High Science (if you were able to stay awake)?

Gasoline at a warmer temperature has expanded, therefore you have less 'potential energy' available than if it were cooler.

Truckers were measuring the temperature of their diesel last summer down here in Arizona to get an idea about the mileage they would get from the volume of fuel they purchased. Google 'Hot Fuel' to get a look at the controversy.

Beware: if you live in an area where it gets quite warm (like here), you will get your blood pressure raised. One of these days, I'm going to open a torch and pitchfork concession.