Fuel Tank Guage on Dash seem Non Linear Is this True?

The actual capacity of the gas tank of the Prius is probably a little less than 11.9 gallons because of the bladder, imprecise fill-ups, etc. I've read a number of posts on here that lead me to conclude that the early pips seem to "last longer" than later ones for some reason. I'd say that my car is pretty good, the pips would appear to represent approximately a gallon's worth of driving.

I've owned very few vehicles where the gas gauge was real accurate across its entire range. But my Prius is not too bad.

 

:target:I just got thru writing you a good little note and my internet pitched me right off line - OK let's do this again: DO NOT try to figure out the gas guage - the whole thing is a relative concept. Because of the inner-tank bladder, full is approximate, as is every step of the way down to empty. Speaking of empty, when you get to one pip (some don't wait that long), especially if it beeps and blinks, hussle it down to the station. There is no promise as to how many miles are left. Take it from someone who has run out of gas (how embarassing!) The car's idiosyncrasies are part of its beauty and personality. Have fun!

 

H,

yeah, i heard that you do not want to rely on gas gauge too much.

i wonder when are other people refilling their priuses: at one quarter, at one peep? ?/

 

It's a GUESS gauge!

 

Every car I've ever had had a non-linear fuel gage, even the old type with mechanical pointer. The problem is that Prius drivers seem to obsess over the mpgs, and try to sweeze every mile from the tank, which is impossible, due to the fact that all the fuel capacity isn't available. Just figure the useable capacity as 9-10 gallons or so, and fill up at the second pip, and you'll be fine.

 

I second what subarutoo just said. I never let my tank get lower that two pips. Also I use the slowest fill and never do any topping off after the thing clicks off. Lifetime mileage has nothing to do with any one given tank's mileage and I never push it past the two remaining pips.

 

I answer this question about three times a week. There are numerous threads devoted to this issue, so I won't go into it in detail. The fuel gauge is non-linear, as it is on almost all automobiles. With the Prius, it hangs around at the top for a long time, then drops fairly fast after that. As for the tank capacity, refer to my post in this thread:
http://priuschat.com/forums/knowledge-base-articles-discussion/44541-ran-out-gas-2.html

A quick search will turn up many more.

Tom

 

Its worse than non-linear, its dynamic

My understanding of the whole system is still a tad fuzzy on details, but I'll have a go. Due to the whole bladder issue, the actual volume of the tank is variable. It will certainly be less than 11.9, I personally have only seen 10 gallons go in once, usually I'm under 9 even when the last pip just started blinking. Since the tank volume is not fixed, the Prius uses data from a variety of sensors to try and figure out just how much gas is in there and how that relates to the current size of the tank. I believe it uses that ratio to determine the % full and decide how many pips to light. For this reason, a pip on any given day will not be equal in terms of volume to a pip on any other day. My guess is that this calculation is least accurate at full, and most accurate as you approach empty. For that reason they "peg" the full reading somewhere above 10 pips, which results in the top pip frequently taking ~100 miles to drop. As the tank gets down a few pips, it starts getting a better idea of where it is, and starts adjusting its estimate. Sometimes this will seem to make the middle pips move faster. By the time you get down to 2-3 pips I think it usually has a pretty accurate measurement and is well behaved. While a few people have managed to travel 100 miles on a blinking pip, most folks seem to run out at ~40-50 miles. None of this is exact, so you are better off just filling up before you get to the blinking pip (or immediately after) rather than risking running out of fuel. This is all largely conjecture on my part, but seems to match up with most of the behaviors I've seen or heard about.

There are certainly times when you need to have some idea how far you can go on a tank. In that case, here's what I try to do.

- Fill up just after the 1 pip starts to blink.
- Note how much you put in. You know you have at least this much gas. For me its usually ~9 gallons.
- As you drive, estimate the gallons remaining as the pips/10 * gallons put in. For the case above this would be ~0.9 gal/pip, maybe a little less if you count the top pip as 2 pips and do pips/11 or 0.81 gal per pip.
- Multiply the gallons remaining by your expected mpg to estimate distance to empty

This should give you a pretty conservative estimate, leaving you a probable reserve of whatever was left in the tank when you filled up. This is _much_ safer than trying to estimate based on how much gas you _think_ the tank should hold.

I think part of the problem is that people assume since the Prius's gas gauge is digital and divided into 10 parts that it is somehow more accurate than a normal car with its analog gauge with 4 parts. In reality its probably no better or worse, although with all the Prius's gadgets it is a little suspicious that they did not include a distance to empty calculation

Rob

 

I have asked the question many times on PC,
"Who has run out of petrol with 2 bars showing on the fuel gauge?"
I am yet to have anyone say they have.

The fuel gauge in the Prius is good, unfortunately people over think things. The handbook says it isn't a good idea to allow the fuel tank to get below 1/4 full, just like every other manufacturer. It also says if you have a single bar flashing at you fill up ASAP. Don't over think it, keep a 1/4 of a tank always or because there is no half bar to show 1/4 tank as a compromise fill up at 2 bars left.

Once you only have 1 bar on the gauge you have no idea how much fuel you have left, could be 10% (there are 10 bars so 1 is 10% but it could be 5% when the flashing starts. Once flashing starts you could have anything from 5% down to 0% left. All cars have some fuel in the car you can't access too so there will be a small amount of tank capacity you can't access. Fill at 2 bars.

 

Last night coming home I had 3 pips. Starting out this morning I had 2 pips. Within 35 miles I had 1 pip. It started flashing a few miles from the gas station close to home. MFD states 486 miles, 55.3mpg. I pumped 8.378 gallons at the slow setting until automatic shut off for 58.0mpg calculated. How much gas was left in the tank? Nobody knows. That's why it's the GUESS GAUGE.

 

On the other hand this is not really unreasonable. If you were showing 3 pips, theres nothing to say you weren't right on the edge of going to 2 pips. The temperature difference of a morning start probably tipped it over the edge. If you use my estimate from the previous post of 0.8 gal per pip at 55.3 mpg you would have expected about 44.2 miles per pip. If your last fillup was similar to this one, at ~8.4 gal, you would be looking at more like 0.76 gal per pip, or 42.2 miles per pip. Lastly, your cold start AM mpg was probably lower than your 55.3 average, so something a little less than 42 miles on a pip would be expected. Given all that, 35 miles on a pip was pretty reasonable. You don't mention how many miles you traveled on the last solid pip, but based on the above somewhere around 40-45 miles would have been expected.

I'm not saying that the gauge doesn't take me by surprise sometimes too, but when you go back and look at it what happened was usually not unreasonable. I think the problem is with our perception of how accurate it should be. I've seen regular gas gauges seem to drop almost an 1/8 of tank overnight, temperature/pressure makes quite a bit of difference even without the effect of the bladder. My wife ran out of gas in our Subaru 10 miles after the fuel light came on. Other report being able to get 30-40 miles, and I've personally gotten over 20 miles. Since the Prius gets 2.5 times better mileage, all these numbers would be 2.5 times bigger to maintain the same relative accuracy.

Rob

 

When it gets to two pips, fill-up!!!!