Prius's MPG is not accurate

[kallusin]

I can not find the email thread that I had with Toyota Support at the moment; but, they admitted the gas gas is NOT accurate. There are only 4 sensors in the tank, none of which are at the bottom of the tank. The gas gage number of bars is calculated using the average MPG.

ALSO, the calculated MPG is WRONG. Case in point: yesterday reset the MPG calculation and then drove 386 miles and used 9.23 gallons (as recorded at gas station when I filled the tank as much as I could). 386 / 9.23 = 41.8 MPG; BUT, the MPG on the car showed 45.6 MPG.

Also, a couple months ago, the last bar was blinking on the gas gage. I added 3 gallons of gas (because I was at a station where the gas was expensive and wanted to hold off filling the tank until the next day when I would be at a station that was 20 cents per gallon less). When I started the car, the last bar remained blinking -- AND IT REMAINED blinking until I filled up the next day.

Two weeks ago, again I was low on gas (bar blinking) and added 4 gallons. Since the tank is 11 gallons, 4 gallons is less then half; but, the gage showed over a 1/2 tank after adding the 4 gallons.


If you are seeing any of this, I HIGHLY SUGGEST you log into the Toyota site and file a report immediately. You should demand that Toyota fix the gas gage to be accurate (until now have you ever heard of a car that does not have an accurate gas gage ?). If enough of us do so and Toyota still refuses to fix the problem, we should begin and report campaign to the US federal government and demand they force Toyota to recall the cars and fix the problem.

As soon as I find the Toyota email (my email search is not working right now), I will post a copy of it.

...david (dcooper000@cox.net)

[kallusin]

I can not find the email thread that I had with Toyota Support at the moment; but, they admitted the gas gas is NOT accurate. There are only 4 sensors in the tank, none of which are at the bottom of the tank. The gas gage number of bars is calculated using the average MPG.

ALSO, the calculated MPG is WRONG. Case in point: yesterday reset the MPG calculation and then drove 386 miles and used 9.23 gallons (as recorded at gas station when I filled the tank as much as I could). 386 / 9.23 = 41.8 MPG; BUT, the MPG on the car showed 45.6 MPG.

Also, a couple months ago, the last bar was blinking on the gas gage. I added 3 gallons of gas (because I was at a station where the gas was expensive and wanted to hold off filling the tank until the next day when I would be at a station that was 20 cents per gallon less). When I started the car, the last bar remained blinking -- AND IT REMAINED blinking until I filled up the next day.

Two weeks ago, again I was low on gas (bar blinking) and added 4 gallons. Since the tank is 11 gallons, 4 gallons is less then half; but, the gage showed over a 1/2 tank after adding the 4 gallons.


If you are seeing any of this, I HIGHLY SUGGEST you log into the Toyota site and file a report immediately. You should demand that Toyota fix the gas gage to be accurate (until now have you ever heard of a car that does not have an accurate gas gage ?). If enough of us do so and Toyota still refuses to fix the problem, we should begin and report campaign to the US federal government and demand they force Toyota to recall the cars and fix the problem.

As soon as I find the Toyota email (my email search is not working right now), I will post a copy of it.

...david (dcooper000@cox.net)

[redrockprius]

Originally Posted by kallusin

(until now have you ever heard of a car that does not have an accurate gas gage ?).
...david (dcooper000@cox.net)

Yep, after almost 60 years of driving and owning about a dozen cars, I've had personal experience with inaccurate gas gauges in -- let's see -- all those cars. To my knowledge, there has never been a car produced that has a truly linear gas gauge that indicates accurately the amount of gas left in the tank.

The bladder in the Prius gas tank just aggravates this situation. Check this forum and you'll find lots of reports about the "faulty" gas gauge. The easiest solution is to fill the tank when one or two bars are showing and stop being obsessive about whether or not the tank is really filled to its 11.9 gallon capacity when the pump stops pumping.

[kallusin]

Sorry; but, I disagree. I have been driving for over 30 years and have never had a car where the gas gage was as in accurate as the Prius's. I have driven my cars many a time to the point where the gage is on the E and when I filled the tank it would fill the expected amount (tank capacity less overflow).

There are only 4 sensors in the tank and none are positioned to indicate when the tank is near empty. That is very poor design and should be fixed.

Other then this problem, I love my Prius.

...david

[madler]

Originally Posted by kallusin

ALSO, the calculated MPG is WRONG. Case in point: yesterday reset the MPG calculation and then drove 386 miles and used 9.23 gallons (as recorded at gas station when I filled the tank as much as I could). 386 / 9.23 = 41.8 MPG; BUT, the MPG on the car showed 45.6 MPG.

Actually your measurement of the 9.23 gallons is the most inaccurate of the measurements you are comparing. The amount of gas you put into the bladder tank ("as much as you could") is a weak measure of the gas consumed by the engine since the last time you put in as much as you could into the bladder tank. The bladder tank changes in volume, and the pumps differ when they decide to stop pumping.

I have carefully tracked for the last 2.5 years my miles, gas, and total gas mileage per tank as reported on the display. I compute the total gas that the car thinks it has consumed by dividing the miles per tank by the gas mileage for the tank and adding them up. I then add up the total gas that I have put in the tank. Those differ by 4%, with the gas I put in greater than the calculated gas consumed. While 4% is not an unreasonable error, I suppose it could be better. But then part of that may be evaporation of the gasoline, and I know a little of the error is from the few times that I overfilled the tank (effectively pressurizing the bladder) and the car then regurgitated excess gas right back out the fill hole onto the ground.

If I look at individual tank fillings, I see variations of +/-15% or more between the calculated gas consumed and the gas pumped. That's why an example for one tank is not meaningful.

I don't doubt that the gas tank sensors are not accurate. In fact I'm sure they aren't accurate. But they are not used to compute the mileage on the display, nor the total miles, nor the amount of gas I put in the tank. Their only value is in warning you that it might be time to put more gas in the tank. A fuel flow sensor is used to determine the amount of gas consumed by the engine. That's the thing that's apparently off by a few percent.

In general, all tank gauges are inaccurate. I fly small planes, and my flight instructors point out that the only time you should believe the fuel gauges is when they read near or at empty. That means you should land very soon. Any other indication is not a good measure of the amount of fuel you have. For that, you have to rely on knowing how much fuel you put in and what your fuel burn rate is based on your manifold pressure, rpm, and the tables in the pilot's operating handbook for that plane.

Another error source doesn't show up in your comparisons at all, since both the miles traveled and the miles part of mpg used the same measurement, which is a factor applied to the number of wheel rotations. As has been noted on this forum before, international regulations require that the error on automobile speed and distance measurements be positive. That is, the manufacturer should assure that it never reads low. That biases the errors to show more miles traveled and higher speeds, and in this case higher mileage than actual.

[bwilson4web]

Hi David,

Originally Posted by kallusin

I can not find the email thread that I had with Toyota Support at the moment; but, they admitted the gas gas is NOT accurate. There are only 4 sensors in the tank, none of which are at the bottom of the tank. The gas gage number of bars is calculated using the average MPG.

ALSO, the calculated MPG is WRONG. Case in point: yesterday reset the MPG calculation and then drove 386 miles and used 9.23 gallons (as recorded at gas station when I filled the tank as much as I could). 386 / 9.23 = 41.8 MPG; BUT, the MPG on the car showed 45.6 MPG.

Also, a couple months ago, the last bar was blinking on the gas gage. I added 3 gallons of gas (because I was at a station where the gas was expensive and wanted to hold off filling the tank until the next day when I would be at a station that was 20 cents per gallon less). When I started the car, the last bar remained blinking -- AND IT REMAINED blinking until I filled up the next day.

Two weeks ago, again I was low on gas (bar blinking) and added 4 gallons. Since the tank is 11 gallons, 4 gallons is less then half; but, the gage showed over a 1/2 tank after adding the 4 gallons.

If you are seeing any of this, I HIGHLY SUGGEST you log into the Toyota site and file a report immediately. You should demand that Toyota fix the gas gage to be accurate (until now have you ever heard of a car that does not have an accurate gas gage ?). If enough of us do so and Toyota still refuses to fix the problem, we should begin and report campaign to the US federal government and demand they force Toyota to recall the cars and fix the problem.

As soon as I find the Toyota email (my email search is not working right now), I will post a copy of it.

You've rediscovered the 3 gallon, refill, reset limit. It takes at least 3 gallons to get the fuel display to reset to what it thinks is the current tank level. In theory, I could drive for years by just adding two gallons per filling as an anti-hijacking defense.

As for some sort of mass action to get Toyota to 'fix' the problem, you might want to look at the Wiki article on USA Prius sales, "over 512,000." This problem has been there since the first Prius was sold about seven years ago and exists because the flexible fuel tank bladder does a better job at controlling gasoline fumes.

Just because I'm curious, how much would an accurate fuel gauge be to you? Are you in the market for an accurate Prius fuel gauge? For example, would it be enough to know when you are down to your last gallon or do you need to know exactly, at all times, how much fuel you have in the tank to say the nearest 1/10th gallon?

I always know when I'm down to my last gallon by putting a one gallon spare can of gas in my trunk when the car gets to the last 1-2 bars. When the car runs out of gas, I drive to a safe area, refill with that last gallon and know I'm on my last gallon. This costs less than $5 for the container and whatever the current price of gas is in your area.

If you want to know the exact amount of fuel, I've thought about removing a fuel tank and re-installing it with load cells. A small microprocessor can read out the weight and report either pounds or gallons of fuel in the tank. But it would cost about $1,200 for the load cells, micro-processor, software and installation.

The other alternative is a custom built, gas tank. However, it wouldn't have the excellent safety features of the current tank and would leak gasoline fumes. Still, it would eliminate the bladder and the gauge would be very accurate. This would run about $2-3,000. It might be cheaper to try and import a replacement tank from Japan or Europe. It would be wise to ask some of their forums if their gauges are also inaccurate.

GOOD LUCK!
Bob Wilson

[madler]

Note to moderators: this thread is in two places in the forum, here and under fuel economy. They should be merged.

[okiebutnotfrommuskogee]

Not a big deal!!!!!
I fill up when two or three bars remain. Whatever gas it takes to make the pump shut off is "what it takes". Drive till two or three bars are left and fill up again. Not worth worrying about.

[kallusin]

Bob,

I like your idea of keeping the extra gallon container of gas in the car.

I am not a fanatic about the gas gage. When doing a long drive (say through the dessert from LA to Vegas), I do not want to stop when the gage shows 1 or 2 bars and pay an absorbingly high gas price. I would rather be able to get out of the desert to a place where gas prices are more reasonable. On the other hand, I do not want to find myself out of gas in the middle of the desert either.

...david

[fish_antlers]

I agree.. the gauge in the prius is terrible. I have already complained to Toyota and had them fix it under warranty (filled 1/2 a tank and never had it register)...

Next step is to complain to http://www.nhtsa.dot.gov/ to force a recall.