Prius Poor Gas Milage?

congrats and welcome! the gas gauge is very inaccurate due to the 2007 gas tank having a rubber bladder which expands and contracts with the weather. so, no worries there, it's normal. to determine how many gallons you have used, just divide your miles by you 49 mpg average. and keep in mind that the 49 is optimistic, and will probably wind up 45-47 at the pump when you fill up.

2 purple bars are normal from time to time and always in stop and go. it can also go up to full green at times. but mostly will hover in the 5-6 range in normal driving. no worries, you're good. all the best!

 

Haha, I think you know the answer to that.

Consider each bar on the gas gauge equivalent to 49 miles. 10 bars equals 490 miles distance. If you have a full tank of gas and drive 48.9 miles, you would still have 10 bars. The gas gauge would register full. When you drive the last 0.1 mile, you lose your first bar.

Now consider, you have a rubber bladder as Bisco stated that expands and contracts. It expanded and only allowed you to fill up 9 bars worth of gas plus 0.1 miles worth. You turn on the car and you get a full 10 bars. The gas gauge would register full. You drive 0.1 miles and you lose one bar. You drive another 49 miles, you lose the second bar. What happens when you drive 80 miles and average 40 mpg?

P.S. I made up the numbers for illustrative purposes. I just wanted to show you what is going on with a digital fuel gauge.

 

it's always prudent to fill up at two bars minimum, especially with a bladder.

 

I agree to look for a place to fillup when you get to 2 bars, regardless of how many (or how few) miles you've driven.

Cruising range and fill-up amount varies pretty considerably. In cold weather, the fuel bladder holds less and the powertrain is less efficient, resulting in far less cruising range than in warm weather. Oh, you're in Miami, so this might be reversed (due to summer A/C use). Here in Colorado, it's typical to have a 300+ mile range in the winter and 400+ mile range in the summer.

The car doesn't know exactly how much fuel it has. It's kind of like estimating how much water is in a plastic bag that's lying flat, but it does the best it can. It's also why the Gen 2 does not have a miles-till-empty display.

 

Bear in mind that the gas gauge is not linear. So, you might drive 300 mi on the first half of the gas gauge and only 190 mi on the second half.

This is not peculiar to Prius, all gas gauges (in my experience) behave in this manner.

 

I think the gas gauge is linear.

Consider the gauge represents only gallons 2 - 8 on a 10 gallon tank. That means 10 full bars show when you have between 8 and 10 gallons and 1 bar blinking when you have 2 to 0 gallons. It may seem the first half of the tank goes much further but only because few dare to venture far after the last bar blinks and everyone will notice the first bar taking a long time to clear. In other words, you go 300 miles arriving at half tank and arrive at blinking bar only after an additional 200 miles. If you have a strong bladder like a Gen 2 Prius, you'd probably get that last 100 miles with a blinking bar.

And all gas gauges behave in this manner in my experience. The needle is stuck past F after a full fuel up and takes forever before going down. And I never let the needle go past E. But you know the needle does go past E when you shut the car off. May I remind you of Seinfeld.

 

Each to their own.

I like to put some science behind what I say, and it seems that way because it is.

And, yes, I am one who dares (not always, just occasionally in the pursuit of science) to venture far after last blip blinks, but have never run out of fuel to date. Luckily for me I do not have the fuel tank bladder to contend with. Sans bladder, I can tell you that the fuel management is quite consistent and easy to work with on the Prius. I'm sure Gen III owners will testify to this, the Gen III also being sans bladder.

For US Gen II Prius owners, still the best advice is to fill at the 2 blip mark. It is just not worth the uncertainty.

 

I find that the Prius has a really inaccurate gas gauge. I have driven 320 ish miles on one tank which took it to the last blip. (Not blinking) Granted, that was from a fill up in colder weather, but it still seems very inaccurate.

 

It is more that it has an inaccurate tank, thanks to the flexible bladder within, allowing variable amounts of gas each fill up.

 

I bought my Prius last July and was getting 52+ mpg. Now I'm only getting 45-47 possibly due to colder weather, though I didn't think Southern Calif 'winter' would make that much difference.

 

5 mpg seems like a lot but percentage wise, 5 mpg loss from 50 average is the same 10% as 2.5 mpg from an average of 25.

 

is it the fuel?

 

No I always use 76 gas. Will see if the mpg gets back up to 52+ in the spring. I park it outside and it can be pretty cold in the morning when I start the car.

 

no, i mean 'winter gas'. it has less energy.

 

Today I found a shocker at the fill-up. Trip computer said 52mpg, but actual mpg was......... 44.4!!!!!!!
Last time was off only by 3mpg. What gives???

 

Fuel theft? Fuel leak? Big over-fill this time? Big under-fill the previous time? Thieving gas pump?

 

Weather is warming up and I am now getting 50-52 actual mpg again (more or less same as trip computer).

 

Don't trust the pump shut off. The pump will often shut off too early, especially with the variable capacity fuel bladder. If it shuts off early on the first fill, you will end up less than full. If the second fill actually fully fills the tank, your resulting MPG calculation will be way off.