Sorry to waste a post on it, but .. from my experience it's been 0.1 off... and i beleive it's due to rounding error. I've tested 7 different times.. i couldn't fall out of the region. I heard some people reporting 1~2mpg off... Just wanted to know how other cars are doin~

 

How are you calculating the error? If you are tracking total miles travelled since last fillup, then dividing by the number of gallons required to fill it back up again, then I would think the guage is more accurate.

 

I tracked mine for several months, and it was generally .1 higher than my calculation, but one time it was off by 1 mpg. I'd say it is pretty accurate.

 

I've noticed that immediately after a fill up, the Tank Average will climb to an unreasonably high value, often as much as 29 MPG. As the tank progresses, the numbers become more reasonable, e.g. close to my normal 22-23 MPG.

I've checked the Tank Average against pencil and paper many times, and over time the manually calculated value averages about .3-.5 MPG better than the one reported on the gauge.

 

My computer is optimistic. It's been anywhere from .4 to 1.2 mpg higher than when I calculate it the old-fashioned way.

 

The problem with calculating on paper is that you have to assume the pumps at the gas station have perfectly calibrated meters, and you have to assume that you are filling the tank to the exact same level. Neither of these assumptions are accurate. A properly calibrated gas pump meter will read a little low so that they don't get fined by the Department of Weights and Measures (or whatever they are called).

 

Possibly. However, it seems as though some are getting very 'accurate' readings while others are getting low and others are getting high. There doesn't seem to be much of a consensus.

 

As it will be when different people have different techniques with different gas stations. A low mileage, well maintained IS will have very precisely metered injector pulses, and the ECU will know how much fuel it is spaying for each combustion event. As the car ages, and the injectors start clogging up, that may change considerably. We'll have to wait and see.

Actually, if a person is absolutely sure their technique is accurate, then a significant error on the guage might indicate a problem with an injector, primary O2 sensor, or MAF sensor.

 

it's either truncation or round off error... i forgot which one it was... 0.1 i could live with it

 

i feel like i'm in an error analysis class. Which all these variable the basic assumptions is ... hold everything else constant

 

Come to think of it, since the 2IS uses 2 wideband O2 sensors as primaries on each cylinder bank, the ECU may calibrate each individual injector to compensate for clogging over time. For example, it could periodically increase injector pulse width by 10% on a single cylinder at a time to verify that the resulting mixture went 3.33% richer. Maybe I should check with the patent office to find out...

 

I hate to be the village idiot here but are you saying that the vehicle is .1 mpg off when calculating MPG ?
Are you considering driving habits such as WOT and idle time in traffic?

 

Same here. Just like everything else with Lexus, the computer calculated mpg is conservative, I'm consistently .3 - .5 mpg higher when using manual calcs. 95% of the time I'm using the same gas station and the same pump, so I'd say my technique is fairly consistent . . .

 

that's all considered in my calcs... i mean it's pretty easy....
you know how many miles you put on .. (trip odometer)
you know how much gas you filled up... assuming you're always filing all the way up.

there are assumptions made but i've been in 0.1 region..