Accuracy of MPG displays?

[Ms.V.X.]

I know I've seen somewhere on the forums about the accuracy of the onboard MPG calculators in cars, but can't locate it right now.

Seems that they have an error of +/- 7% as I remember...is that right?

My '07 Camry has an electronic MPG calculator, and I will be starting a gas log to compare to it. Just wondering if anyone here has had any real world experience as to how close these onboard displays are?

Thanks!

[theholycow]

It varies heavily from one vehicle to the next.

It's definitely worth keeping a gas log, regardless of an onboard MPG display. You get all the data, it gets put on a graph for you, you get to keep other notes (like maintenance), and it becomes easy to track and compare.

[bobc455]

On my wife's Buick, it's consistently 8% too low and we always use more fuel than shown on the display.

-BC

[benfrogg]

Tire size can play a large role in accuracy. Even if the gauge was accurate to start with, a slightly different tire from stock will reduce it's accuracy. In fact, I imagine new deep tread tires to very worn out tires would have an effect.

I like that my gauge can be adjusted manually. (MPGUINO) That's really the only way to get close.

B

[SL8Brick]

My Audi's display will occasionally show over 200mpg while decellerating/coasting downhill...I've always found that kinda funny.

[theholycow]

It's probably indicating DFCO.

[GasSavers_GasUser]

Originally Posted by SL8Brick

My Audi's display will occasionally show over 200mpg while decellerating/coasting downhill...I've always found that kinda funny.

Yeah, mine too.......downshifted going down the mountain mine reads 9999mpg......it's hilarious ... DEFCO........haha

Then I pay dearly going back up later...(no need to discuss that...haha)

[kamesama980]

agreed, depends on the car, the method they/you're using to read it, calculate it, and display it. check yours at the pump a few times to figure out the margin for error and how to correct for inaccuracy.

[Martinjmpr]

My guess is that most factory-installed MPG displays cannot be calibrated. My 99 4runner doesn't have one but I put in an Ultra-Gauge (which is similar to the Scangauge in that it gets info from the OBDII port and calculates MPG.) I had to calibrate it a couple of times (first for distance and then for MPG) but now it's dead on, less than 1/10 MPG different from my hand calculations.

[GasSavers_Pete]

They do vary a lot as Holycow said.
The one I have in my GM variant is accurate as hand computing and shows down to 0.1 ltr/100 klm.

I had a free (under warranty) upgrade of the electronic unit which makes the actual calculations but it seemed to be just as accurate before as after.

Peter.