Prius's MPG is not accurate

Re: Gas gage not accurate -- Toyota Support agrees

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.

 

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.

 

Hi David,

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

 

Re: Gas gage not accurate -- Toyota Support agrees

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

 

Re: Gas gage not accurate -- Toyota Support agrees

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.

 

Re: Gas gage not accurate -- Toyota Support agrees

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.

 

Re: Gas gage not accurate -- Toyota Support agrees

Just happened to remember, I drove a Town Car for almost 5 years. The gas gage never worked (always said 3/4 full) Used the trip odometer. Filled up at 150 miles in city driving and 250 miles in highway driving. Never ran out of gas.

 

Re: Gas gage not accurate -- Toyota Support agrees

wow... you bought it like that brand new and the gauge never worked and you never once said anything about it because it was under warranty?

You never had a little voice inside your head tell you that it was wrong that your brand new towncar had a busted gas gauge?

 

Re: Gas gage not accurate -- Toyota Support agrees

bull. there's a float gauge in the pickup.

http://priuschat.com/forums/showthread.php?t=30593

 

Re: Gas gage not accurate -- Toyota Support agrees

The only car I've driven that had a very accurate gas gauge (and speedo, for that matter) was my Volvo S80. It was very precise. Otherwise, the gauges have not been that mathematical in my experience.

 

Re: Gas gage not accurate -- Toyota Support agrees

how innaccurate? Like say your gauge read completely empty, flashing warning you to add fuel, and you bought 1/2 a tank, but it still read completely empty?

Like that innacurate?

I think most people expect gauges to be accurate to within reason, but the gauge on the prius is terrible... I mean it can be 100% wrong and any prius owner can replicate this failure reliably.

Surely that points to a problem that needs to be addressed by Toyota....

if not on their own, then with help from the government making them fix it!

 

I find that a warmed-up Prius can easily get 45 miles on one gallon. If you keep the speed at 55 mph, you're looking at 60+ miles. This should be enough to get to the next exit with an open gas station. A two gallon can would double the expected distance.

Bob Wilson

 

Re: Gas gage not accurate -- Toyota Support agrees

Actually it was a used 1988 model. We bought it with 159,000 miles on the odometer and sold it with 213,000 showing. It was still a good car. Sold it when I bought a new 03 Civic Hybrid. I had the Civic for almost 5 years until buying the 07 Prius.

 

Re: Gas gage not accurate -- Toyota Support agrees

so you expect a brand new hi-tech car made in 2007 to come stock with a busted fuel gauge just like the used 1988 jalopy you bought that had 159K miles?

Wow.. warranty people must love you! You're very forgiving and appear to have very low expectations from products!

 

It's convenient that the reading on the consumption display zeros out when the tank is filled (assuming you fill it when adding fuel.) I can rely on it to inform me when it's time to fill up since my lifetime average according to the same display page is 47 mpg. That means I can go 470 miles on ten gallons which is about the amount of gas that goes in at my fill-ups. Sometimes this number matches the gas gauge and sometimes not. The fact remains that each fill-up is pretty much the same ten gallons.

The ten gallon fill-up also has a nice feature for a quick determination of gas price. At $3/gal X 10 = $30. At $3.25/gal = $32.50. So I wonder if it is worth worrying about $2.50 just to find the station with the cheapest gas. I don't.