jaxxx

Our MPG reading on the dash..how accurate isi t?

WeisISF

iTrader: (8)

i would say so so mind say 14.8 mpg lol

thechosun

iTrader: (3)

Its pretty spot on if you have stock rims i think. I dont think it calculates based on GPS because not all models come GPS equipped. that means it has to rely on rotations per minute from the drive wheels and if you increase or decrease your rims and tires, the calculations start to get off...

at least thats my speculation.

KillaIS250

iTrader: (1)

I use an app on my iPhone to keep track of gas mileage. I just have to punch in the details from the fill up and it calculates it everytime i fill up again. I just got my IS250 Saturday, but i've filled up twice already because i've been driving so much. So here's what i got on the last tank:

MPG according to the IS (tank average): 29.1 MPG
MPG according to my app: 29.51 MPG

So not bad, .5 of an MPG off.

lobuxracer

iTrader: (2)

It all depends. Sometimes it's pretty good, sometimes it's terrible. The more starts and stops you have (key switch events) the more likely it is to be wrong. Here's some real world data I collected:

texas02118

iTrader: (1)

In my case my MPG readings are way off and my car is all stock, except for the GFX front lip and redouts lol.

Anyway, I calculate the MPG everytime I fill up my tank and the MPG on the dash is off by 2.0 to 3.0 miles.

For instance,

IS Dash MPG = 30.1
MPG according to my calculation #miles/# of gallons = 28

ping83

iTrader: (8)

Mine was dead on when I did the manual calc. Of trip mileage / gallons of fuel

willfxp

Some pumps could vary about when they cut the fuel, so probably need to make sure you're using the same pump each time or have many samples.

lobuxracer

iTrader: (2)

There are any number of variables you cannot control. Pumps are not calibrated for precision AND accuracy. Most "certified" pumps are tested at a single volume on a single trial. If it pumps 5 gallons on one pass, it's called good. It doesn't mean the pump was right at 1, 2, 3, or 4 gallons, only that it properly filled a 5 gallon test measure. So in reality, the pump could be very wrong over a single fill, but over time (think multiple customers) it's reasonably accurate.

Besides, if we really wanted to buy gasoline in a repeatable fashion, we would need to buy it by weight, not volume. When your engine's air/fuel ratio is calibrated, it's not calibrated to volumes of air and fuel, it's calibrated to masses of air and "expected" masses of fuel. Your MAF reads in grams per minute, not liters of air because measuring the volume is pointless.

This also brings us to how the MPG is calculated - One of the injectors is modeled and monitored to determine how much fuel is being used (which varies by fuel density, but that's just another uncontrolled variable the O2 sensor fixes) and this volume is compared with distance traveled. It's not terribly accurate as algorithms go, but it's not completely useless either. This is part of why you see the wide variation in my spreadsheet for calculated vs. indicated.

Take what you see with a BIG grain of salt. It's not horrible and it's not great, it's just a helpful indication.

ajmtbm

i ahve found the gauge in the car reads 2.5% higher MPG than actual pretty regularly

JMcDonald

iTrader: (2)

I dont know how accurate it is but I know I lost about 90 miles a tank when I put my 20's on.

ISRocker

iTrader: (1)

The computer in the car says "Tank Average 22.4". But when I calculate it by using the odometer reading and number of gallons to fill up the tank I get 20.96 mpg. I'll do it a few more times to get an idea...I usually go to the same gas station to fill up. I try to use the same pump when calculating mpg, but it isn't always available...

JohnLX300

iTrader: (2)

After two months of fill ups.....once a week, I recorded my actual vs what the stupid car says....I've gotten the about the same, the car states I am getting an average of 2.6% better gas mileage than I actual am. I think this is beyond any variable of inaccuracy and blatantly sucks!

nastyemu

i fill up and manually calculate every time using logmympg.com.

usual reading per the car is 23.5-25mpg. actual calculated mpg is closer to 21-22.

knguyenk

My AVG MPG on the display is pretty bang on with 23.7 mpg with AWD and I check with hard calculations with the read on the Trip computer (which is reset before shutting off the car at the pump) and the number of gallons sold on the receipt.

When it comes to the TANK AVG, that's where it varies; it would read 25 mpg but I would get in actually, through calculations, I get around 23-24 mpg. So basically a small margin of error of 1-2 mpg and about 4-8% off.

EDIT: All stock with the exception of F-Sport intake and Tanabe MT exhaust.