Fuel gauge, calculations accuracy

Each squirt of fuel into the piston is only an approximation.

Rounding errors are pronounced the further you drive.

The interface for PHV truncates to make that easier to notice/understand/accept.

 

Accuracy is a problem. Several years ago I gassed up twice at the same pump, and both times my 2004 Prius calculation was way off. So I took a plastic jug and carefully measured one gallon of water into it and marked the level. After drying the jug, I went to the same gas pump and pumped in exactly one gallon (actually I was .002 off) of gas. It turned out the gas level was well below the one gallon water mark. I was paying for one gallon of gas but my Prius was getting less than that.

I complained to Weights & Measures, and they said they did not have enough people to investigate specific complaints such as mine, and the gas station would be checked by them on their regular schedule which he thought was every two years.

 

Well if you take the car's MPG.. 1309/140 you still only get 7.4 gallons
It does truncate, but the error is still pretty significant.

I disagree with john's reasoning for truncating as nothing actually show its doing that. (And rounding would be better if that was the idea).

I would argue that the reason to truncate is for marketing "feel good" effect . By truncating fuel used, it seems like you used less, which feels good. I find it disingenuous that they truncate fuel used to gallons, but then show fuel "saved" to tenths of gallon. They can measure actual fuel used way more accurately than can estimate fuel saved.

 

When my last bar started flashing, I drove two more days before I went to get gas (but those two days only used gas for "warm ups"). I put in 9.1 gallons. I probably clicked it a few times to get to $40. I think the PiP has a 10.6 gallon tank, IIRC

 

No. I've never seen an automotive gas gauge with that sort of linearity. Most are calibrated at the low end, to avoid running out of gas, but not at the high end. Generally a full tank will read as full for some time before the gauge starts moving down, so the gauge is not linear over the full range.

How should you read it? Like this:

Full = Full or nearly full.

Half = Somewhere around half.

One bar = Low; you better get gas.

Flashing = Get gas right now!


Tom

 

One bar = one bar and the value of a bar varies. For a gross approximation, 1 gallon/bar isn't too bad.

Keep in mind, the gas gauge is just reading a crude variable resistor operated by a float in the gas tank, just like any other car.

 

I've had three fill-ups and calculated at the pump was right on with indicated consumption in the car.

540 miles @ 59MPG
w/ 9.15 gallons pumped

722 mile @ 75MPG
w/ 9.61 gallons pumped

820 miles @ 91 MPG
w/ 9.17 gallons pumped three clicks after auto shutoffl!!!!!!

Those are almost perfect calculation results.
I got pics too.

 

When the last bar on the fuel gauge starts blinking, I end up putting in about 8.5 to 8.75 gallons, leaving a reserve of about 2 gallons. When I have driven the car some 30 or 40 miles past when the blinking started, I end up putting in 9 to 9.3 gallons. I have not tried pushing it past that.

 

In checking the linearity and accuracy of the fuel gauge I'm surprised no one has mentioned confirming the accuracy of the estimate of miles remaining to drive too? I think I will do that in a few weeks. I am a little fearful of running the tank dry and only did that once 'accidentally' with my first 2007 Prius. (I knew I was very low on gas and looking desperately for a gas station. Believe it or not when the ice stopped I could still use electric only to limp a block or so into the gas station that thankfully appeared. Once fueled up the ice started right up and I drove off. When I do this test I'll take a 2 gallon gas can and stop and add it about as soon as the estimate miles remaining hits zero. I'm pretty sure that although the estimate with a full tank could be far off that by the time the tank is almost empty the estimate is pretty accurate. The first time I tested this was with my 2010 Prius and it on two tests continued to run close to a mile past the 0 miles left estimate before I stopped and added the gas.

 

Did you happen to look at the estimated miles remaining said when you went 30 or 40 miles past when blinking started? Or what it said right when blinking started?

 

Unfortunately I can't remember for sure. I think sometimes it's said 20 or 30 miles. And at other times it said 0 miles. I'm assuming you are referring to the Distance To Empty (DTE) figure, or Estimated Cruising Range figure. Next time I have to fillup, I'll make a note of it, but that won't be for a couple of weeks.

 

when it says 0 miles left....(DTE) it take into account your EV range too....

IE: 0 DTE.....charge car, you will have 13.9 DTE = 13.9 EV range.

I have gone 43 EV/HVmiles after blinking of approx 20 of which were HV miles, and shown 0 DTE, and only put in 8.6 gals.

I am ready for my second fillup since i got car august 11th.....

I get exactly 1000 miles to a tankful so far.....2000 miles now, will be 2nd fillup tomorrow. Im sure i could go another 100 miles with my 50/50 ratio easily with a blinking light but wont push it.

Dan

First fillup stats...I will add 2nd fillup stats to my sheet tomorrow

 

*sigh*, $3.76 a gallon. Haven't seen that low a price in a long time...

 

A regular Prius has an 11.9 gallon tank. Anecdotal evidence across the cars I've driven tells me that when the gas light comes on, there is about 2 gallons remaining in the tank. So 1 bar per gallon sounds about right for a regular Prius. My owner's manual says the PiP has a smaller 10.6 gallon tank. So that means each bar equals less than a gallon.

Echoing others, the fuel gauge is an inaccurate indicator of fuel consumption. This is because the fuel gauge is calibrated to only show fuel capacity between say 20% to 90% of capacity. The actual numbers differ across different makes and models. But using this example, this means that it will show 10 bars whether you are at 100% or 90% capacity. That first bar seems to take forever to go out but the second bar goes out when you are only at 83% capacity. All the other bars go out linearly every 7% of consumption. Likewise, there are zero bars or DTE = 0, whether you are at 0% or 10% capacity.

Obviously, manufacturers do this on the low end so that we don't run out of fuel driving the car down to empty. On the high end, I don't know why they do it but my hunch is that some people would complain if their first bar goes off quickly or much faster than a neighbor's similar car. Refer to this thread for an example. Seeing that needle not move or that bar unblinking gives a false sense of fantastic fuel economy.

I think the 7 gallons shown on the display is like your odometer and this is pure speculation. The odometer doesn't show you travel 8 miles unless you travel over 7.9 miles. So once you consume more than 7.9 gallons, it will show 8.

When the last bar blinked on my PiP, I filled up 8.343 gallons. Distance to empty read 10 miles. Again, about a 2 gallon reserve. So we filled up a similar amount of fuel at a similar fuel gauge spot. My display mpg was also wildly off but in the other direction. It said 70.3 mpg. My UltraGauge indicated a much closer 79 mpg. I had traveled 668 miles which should be 80.1 mpg. This was also my first fill up. Maybe my dealer did some clicking action during fill up? Who knows, let's see what happens on fill up number 2.

 

yeah that was over a month ago, but now finally after being at 4.09 we are back to 3.75 here where i get mine, this was great cause my PiP rode out the price hike and fall, so i did not have to buy any fuel at those prices!

Dan

 

I just started adding how many Down-To-Empty miles I have left to my Fuelly notes during each refueling.

It might be interesting to organize a public spread-sheet for our trips so we can compare, including data such as:

miles miles(EV) miles(HV) MPG(car) MPG(pump) gallons(pump) gallons(car) kWh(car)

What made me think about this is when I got gas this morning, the car calculated 81mpg, while actual mpg was 75.9. When I refueled last time, the car MPG matched the pump MPG exactly, using the same gas station and pump, but only had 604 miles on the car, so this is rather weird.

PS: Once the pump clicks, don't keep trying to add more. You are wasting gas (gas station gets it back), plus from what I have read, you definitely don't want to do this with the Prius tank.