Gas Mileage/fuel Gauge

Congratulations on asking one of the two most common questions found on PriusChat. Your question has been answered innumerable times, so a search would be in order. Try searching for "guess gauge".

The short answer is that the fuel tank capacity varies due to the internal fuel bladder. Don't expect the fuel gauge to be linear.

Tom

 

since you only put 7 gallons in expect to only go about 350 miles at a 48.7 mpg average. The fuel guage is called a guess guage for a reason.

 

I have pretty much the same situation when it comes to filling up the tank for a little over 7 gallons and getting a lower than expected mpg reading on my MFD. My mpg has been going up and down from 48 to 51 mpg. Lately, I try not to worry about this too much and just drive my Prius with fuel economy in mind or else it will start to drive me crazy. :fear:

 

Yes. Please do not capitalize every single word in a sentence.

Thank you.

 

Changes of 5% or so are pretty normal and are due to the numerous variables we all encounter on the road.

... Brad

 

Welcome. I presume you are from the other end of the state. Situation normal. The Guess Gauge isn't any better or worse than the others. Try pumping slowly and don't top off. Running the ac at 79F with outside temps 80+ doesn't hurt the fuel economy. You can get a few more mpg by inflating the tires and blocking the grill.

 

I'm not sure your questions are fully answered here. I, too, have noticed a change in the guage from earlier times with my 2006. It used to be 45 to 55 per pip, now is 25-35 per pip (after the first one goes down at 130 or so). I have been calculating mileage since I noticed the change, and it seems to be fairly close to the computer mileage though. So I don't know exactly what is going on, except that a change has occured and I liked it better before...

Beyond that, all comments are true, the tank can be difficult to fill consistently being a particularly annoying fact (one of the few annoyances I can find about the car)

Jim

 

I know this seems to be a common question, and I apologize for reposting, or asking again but here goes....I got my 2008 Prius, touring package 6, on Sept. 2nd and finally filled up the tank for the first time. It was down to the last bar and I had gone 419 miles, so I though it was time. I was only able to fill 6.5 gallons of gas and it reads full. I'm confused because on the Info screen I was supposedly getting 43mpg. Somehow, all this doesn't calculate correctly. Can someone help me figure out what I'm missing?

 

Congratulations on your new Prius! There are a few possible explanations for your observation. You're correct that the numbers don't add-up (or divide out). One explanation is that the dealer may not have fully filled your tank -- perhaps even without poor intentions. I have about 1500 miles on my 2009 Prius and typically get around 100 miles on the first pip (rectangle) on the computer display, so you may not even have known it wasn't completely "full".

The Prius has a different design to its fuel tank than most vehicles to reduce gasoline vapor emissions. Rather than a rigid tank, the Prius has a flexible bladder. Because it is flexible, the amount you fill into the tank will vary as it stretches to accommodate the fuel you place in the tank. For this reason, it's probably best to fill the Prius tank slowly (the slower setting on the pump handle). At colder temperatures it tends to be more rigid and probably will hold less than it will at higher temperatures.

I suspect as well (from my own experience) that the first filling of the tank is less full as it hasn't been stretched significantly, yet, and is still a bit more rigid. This could be a reason why the dealer's filling wasn't truly full. The bladder didn't fully stretch, and the tank "appeared" to be full.

I suspect your next few fillings will get progressively closer; though, your calculations will probably never exactly match the computer due to the variation in bladder "capacity". Other posters have indicated the computer is actually reasonably accurate.

Hope this helps, and I'd be interested to see how your subsequent tanks turn out.

Happy driving!

 

Very interesting posts. I have a 2008 Prius with 2200 miles on it. Today, after driving 450 miles I decided to fill up with the gauge at 2 pips. It only took 2 gallons before the pump shut off. I tried another pump, and forced in another two gallons. The fuel gauge now shows four pips; can't get it to take any more gas. I'll try again tomorrow when everything cools down (including me). From my readings on this forum, this problem is inherent in the tank design.

 

It could also be the pumps at the service station. I'm sure that we both doubt that you are getting 110mgp. Some members have posted that they have more problems at some service stations than others. Different sensitivity of the pump's cut-off mechanism. It may be worth it to try a different station.

There should be other replies from those members more knowledgable. Some posts have seemed to imply that the tank/edu system may have some type of memory that takes several tank refills to reset itself. :noidea:

Good Luck.

 

Yes, there's something about the tank and fill pipe design that brings out the shut-off sensitivity in gas pumps. If you have a problem with one, try going slower, or try another pump.

 

Here's an attempt at a simple quick description of the process. The filler tube actually has a seal that needs to make a seal with the pump's nozzle. It's purpose is to prevent any gasoline fumes in the bladder from escaping back out of the filler tube as you fill the tank. The fumes are forced through a carbon filter and also stored in the tank outside of the bladder and later fed to the ICE to be burned. If the tank is filled with fuel fumes it will restrict the amount of fuel the tank will allow for a fill-up. If the ECU fails to sense a good seal, lack of positive pressure, or fumes escaping, it will close a valve preventing further filling. If it senses too much back pressure, true or false, is will also close a valve preventing further filling. If the station pump is too sensitive, the positive back pressure may prematurely cause the pump to click off. This is all done to qualify for one of the environmental ratings of the Prius.

Disclaimer: This is only how I understand it to work and more knowledgeable members are more than welcome to add and/or corrrect my assumptions.

Important: This is also why you do not want to try forcing more fuel into the tank. If it's the Prius' fuel system that's stopping the fill-up, you can force fuel down the vapor vent in the filler tube and into the tank outside of the bladder and perhaps cause damage to the system.

So net results;
Filling the tank too quickly can cause increased back-pressure, triggering the Prius' shut-off valves and thus making the pump click off. So use slowest setting or try another pump or station. Or try again later giving the fuel system a chance to stablize.

If problem persists check PC for other Prius owners in your area and try one of the stations they use.

If it is still a problem, you need to have the filler tube seal checked for damage and have the fuel system checked out. It is an extremely complex system with multiple sensors, compartments and valves, directing the fuel and fuel vapors through a number of different paths and valving, under different circumstances, including pumping fuel vapor to the ICE.

This is probably more than most of you want to know. But I've always found it helpful, if I have at least a basic understanding, of why I should or should not be doing something and how it works.

Good Luck!