very specific fuel economy question (2005 model)

Hello Dan, welcome to PriusChat

There are many threads on here on this exact subject == just use the search feature to find them.

Briefly, the problem is that the fuel tank has a bladder which changes with temperature and makes the gas guage very inacurate. It is also not linear due to the shape of the tank.

Your logic is good with a typical tank, but not with the Prius.

 

I think your problem is that you think your tank is minimum 10 gallon capacity. Not sure where you got that from, but your tank can be anywhere from 5 gallon to 12 gallon. It just depends on the temp due to the bladder effect.

 

I think the tank is 11.5 gallons. It's stated in the manual and in a few other posts. There are also a slew of posts about the guess gauge and the bladder that makes up the fuel tank and how inaccurate it can be.

On your last trip where it tells you to fill up, how many gallons did you put in previously? The most I've EVER put in has been 9.81 gallons. So I would have known I needed to fill up somewhere after 450 miles of driving.

Now, is 400 miles that bad? I didn't get that much in my Jeep that had a 23 gallon tank! I certainly don't think anything is wrong with the car, so you have two choices.

1. Go by what you believe to be true - you can go another 70 miles. Maybe you'll be able to, or maybe you'll run out of gas.

or

2. Fill up when the car tells you to or, better yet, before it tells you. The most miles I've driven before filling up was 436. The least was 194.

 

Had a very similar problem on our last trip up to Lake Tahoe. I was hoping to make it up to the lake and then run down to Carson City for some errands and fill up. We had about 420 miles on the tank as we got to the lake and the last bar started to flash. Our tank average was about 47 MPG. Put on another 10 miles at the lake so a <30 mile trip to Carson City, should not have been a problem. But, I wimped out and put in a few $ at lake prices (+ 35 cents/gal) before making the trip down. The problem is you just don't know how complete your last fill up was due to the bladder. Also, it's not clear that there is any direct relationship between the bar starting to flash and actual quantity of fuel in the tank. I probably could have made it but the risk of running out of gas at night in the mountains just wasn't worth it.

 

No, you don't. But at a minimum, you at least know how much you last put in, so you do have a pretty good idea as to what your failsafe distance should be.

 

Our FAQ also might have some insights for you.