i have a 2009 prius. i don't think toyota has ever really done any software update on my car. is there software updates that need to be done on the car? i've noticed my car hasn't been accurately been reading my MPG after each fill up (i do the math on paper)
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?
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
sorry if i wasn't to specific earlier. I've been writing down my Odometer, Gallons filled, Price of gas, and total cost, along with the date, on each fill up i've had with my car since August 18th, 2009. After I fill up my car at the gas station, the second i turn my car back on, i reset the MPG and miles driven on my display screen. I have done no mod's on my car. The only things that are different are the tires because i got a flat tire once, took it into toyota and got a new one. I haven't attached anything to my car, and i haven't messed with one thing in or on my car, besides getting new floor mats Unfortunately I have only been keeping a record of my manual calculations and not the ones on the car. I'm going to start doing that on the next fill up though, but my last two manual calculations i got 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 those are just the last two, i can't remember what the car read on ones before that, but they've started to be off slightly starting around 2 months ago, but i had no major updates or changes on my car at all at that point, it's just been a gradual change that's been sneaking up on me week by week. @bisco not sure what that has to do with it. maybe you're thinking i'm saying i'm not getting as much MPG than i use to or something, but that's not the topic at all here. it's just that my car is calculating a different MPG average than my hand written calculations for each fill up i do. so is there any kind of software update that needs to be performed on my car by toyota's dealership? I assumed that since it has software in my car, software, by its nature, occasionally needs to be updated to fix any kind of bugs. is this something toyota does not do to prius cars? i'm asking you guys on here because i trust the people on these forums more than the mechanics at the dealership sometimes. example: i found on here how to turn off the beeping sound my car makes when put in reverse. but when i took it into the dealership before that and asked them if it was possible to turn the beeping sound off, they came back to me and said that there is no physical way to turn it off. so i trust you guys a little more
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.
so are you saying that they may be different because one week i filled up when i was only half way empty with gas, and maybe the other week i filled up when i was 3/4 of the way empty?
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.
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.
i check my tires maybe about once every 2-3 months. not sure if i should be doing it more than that or not. i can't remember what they're at right now, but i do try and keep an eye on that though cause i know it makes a big difference
