Gallons used vs Gauge

PriusMikeAZ · 02-08-2008, 05:10 PM

I drive the car until the last tick on the fuel gague blinks.

I used to fill up and it would take a little over 10gallons. Now I fill up and it's taking 8.3 gallons or so.

My computer says I'm getting 47 or 48mpg.. I figure it by hand it says 44 or 45mpg.

What gives? I don't understand this bladder thing. What is the purpose of making a fuel tank that expands and contracts?

Is this just a "winter" thing and in the summer I will have more "room" in my tank?

Am I really using less fuel now? Is there something wrong with the fuel gague?

2006 Prius
13.000 Miles
Phoenix, AZ
- Mike

qbee42 · 02-08-2008, 05:49 PM

This issue has been posted and commented on dozens of times. A quick search will give you a lot of reading material, but I'll recap it here:

1) The bladder is there to reduce the amount of gas evaporating, which in turn helps keep down emissions.

2) The amount it expands and contracts varies with temperature and other factors.

3) The mileage on your MFD is accurate. What you calculate is not. Since you have no way of knowing how much fuel is really in your tank, you have no way of doing manual mileage calculations. You can do manual calculations if you do them over a LARGE number of tanks. That way the uncertainty of the bladder gets factored out.

Tom

02-08-2008, 05:10 PM	#1
PriusMikeAZ Junior Member Join Date: Jul 2007 Posts: 3 My Car: Model: Package: Thanks: 0 Thanked 0 Times in 0 Posts Friends: 0	Gallons used vs Gauge I drive the car until the last tick on the fuel gague blinks. I used to fill up and it would take a little over 10gallons. Now I fill up and it's taking 8.3 gallons or so. My computer says I'm getting 47 or 48mpg.. I figure it by hand it says 44 or 45mpg. What gives? I don't understand this bladder thing. What is the purpose of making a fuel tank that expands and contracts? Is this just a "winter" thing and in the summer I will have more "room" in my tank? Am I really using less fuel now? Is there something wrong with the fuel gague? 2006 Prius 13.000 Miles Phoenix, AZ - Mike

02-08-2008, 05:49 PM	#2
qbee42 My other car is a boat Join Date: Mar 2006 Location: Northern Michigan Posts: 17,717 My Car: 2006 Prius Model: N/A Package: #7 Thanks: 1,353 Thanked 2,791 Times in 1,874 Posts Friends: 19	Re: Gallons used vs Gauge This issue has been posted and commented on dozens of times. A quick search will give you a lot of reading material, but I'll recap it here: 1) The bladder is there to reduce the amount of gas evaporating, which in turn helps keep down emissions. 2) The amount it expands and contracts varies with temperature and other factors. 3) The mileage on your MFD is accurate. What you calculate is not. Since you have no way of knowing how much fuel is really in your tank, you have no way of doing manual mileage calculations. You can do manual calculations if you do them over a LARGE number of tanks. That way the uncertainty of the bladder gets factored out. Tom __________________ *Black* 2006 package #7 Northern Michigan