MPG reading not accurate anymore

how much has it changed since you bought it?

 

Have you done any mods, like Tires, Wheels? If so, that would affect the MPG. The +12 battery should be fine, but it's worth checking into. There's always a chance it is ready to be replaced. When it goes bad, you get bad MPGs. Any new drivers that are not Prius friendly? What MPGs were you getting at best, what are your MPG numbers now?

 

it's the blatter.

 

Please be more specific.

What is the MFD displaying and what do your manual calculations say? (No, not just w/1-2 tanks.) How large are each of these fills? Can you give us some examples? Have you been tracking the numbers from both along w/the fill sizes in a spreadsheet to see if they converge?

Are you resetting the MFD between each fill? If not, you should, assuming you want to compare the MFD's values for that tank to your manual calculation.

 

Hi Brady. After my 12V battery was disconnected one time my displayed MPG was out for a while. It got better over time but it took quite a few weeks before it was totally back to normal. Apparently it has something to do with stored historical data that the computer uses to help calculate MPG accurately.

 

Manual calculation on one tank is nearly meaningless and not worth worrying about. Do it for a combined 10 or 20 tanks and it will be much more accurate.

 

The OP has a NA Prius with integral fuel tank bladder. This means that his manual calculations are inherently flawed.

If you want to compare manual calculations with the ones from the computer, you need to do it over a large number of tanks. The numbers should converge pretty well after about 10 tanks.

Tom

 

Hi Brady. The thing that immediately stands out about your data is that the average of your calculated values looks remarkably similar to your MFD (multi function display) indicated values.

This really does look like a simple case of slightly inconsistent fill volumes from one tank to the next.

 

No not necessarily. What I meant is that each time you fill up the exact point at which the pump cuts out can vary a little bit from one time to the next. Sometimes you'll squeeze a little more fuel into it and sometimes a little bit less. This is an issue in any car but apparently much more so in the North American Prius because of the tank bladder.

 

if you are thinking the display should match your math, you are incorrect. there is no software update, it's true of all our cars and toyota and most other manufacturers are not interested in fixing it.

 

I have kept a spreadsheet of calculated gas tank MPG vs MFD displayed MPG for the 120K that we have had our Prius. While the overall average calculated MPG is lower than the displayed MPG by about 2%, I have found individual tank variations of up to +/-21%. As mentioned, the North American 2Gs have a bladder in the tank and that bladder exerts a back pressure. That, coupled with variability of the pump shut off point, can make individual tank calculations way off. The problem is much worse in cold weather, when the bladder is stiff and exerts more back pressure.

 

i have the same problem with my bladder.

 

When we first got the Prius I would calculate the fuel consumption each tank full. Since we don't have the bladder here I found that I could get reasonably consistent results if I filled the same way each time, stopping the first time the pump cut-off etc. I was averaging about 54 MPG, but the individual tank results were pretty close, like mostly between 52 and 56 MPG.

After a while however my wife starting filling up and all of a sudden the tank averages started going like 45 MPG one tank and then 70 MPG the next (even though MFD still showed about 54 MPG). Clearly the level that she was "filling" to was somewhat more erratic than I was doing.

 

Search the site for "TSB", or google it. But I don't remember seeing any SW updates in the last couple years.

What are the tire pressures right now, before you drive anywhere, and how often do you check them?

Wait. Is this what set you off?
"43.39MPG April 3rd
37.88MPG April 10th

but on the car, it told me for the first one, somewhere around
40MPG April 3rd
41MPG April 10th"

That discrepancy is nothing, a matter of gas pump shutoff, pure happenstance. The wonder is that you had better agreement than this for two years! Keep driving and forget about it.