newbie: Is the display true?

[RMinNJ]

I zeroed the display when I got gas ( 3/4 tank Shell Premium (93) gas).
So my first 2 trips to work, no resetting of the MPG display;

40.8 MPG
40.4 MPG

Is this true? It dipped to 37-38 on the way home yesterday but climbed today.

My trip is a mix; Going: stop and go to one suburban road (which has a lot of downhill where the MPG climbs) , then the interstate which is stop and go at some points. Return trip (different) is 2 stop/go highways and a 5 mile free-for-all interstate stretch (kept it at 70-75).

My new (used) 2012 LT, 15inch wheels, 1.4T Sedan 6AT.

 

[ahtoxa]

It's not totally accurate. Most people report that the readout is optimistic by 1-5 MPG.

 

[Dubbs_RS]

Mine has been just about spot on every time I do hand calculations. Typically within about +/-1. This last fill up mine read 28.3 and I calculated 28.27 (I know... bad mpg... I have an auto RS, a lead foot and a bad commute :/).

 

[laborsmith]

On the LT I found the display optimistic when compared to hand calculation but over the course of say a thousand miles of driving pretty close. On the RS I have never changed the readout but it shows 34.4 with nearly 25,000 miles probably 80% highway. I suspect that is pretty close but I am not OCD enough to keep that accurate of hand calculated logs to say for sure.


Laborsmith

 

[Swordsmith]

I never reset my display, hoping to get a lifetime average. Over the course of a year and a few months it gradually settled to 27.2 and rarely varied much. A couple months ago it shot up to 38.6 and has only slowly dropped to 38.2 over the course of several tanks.

The first number may be accurate. I tend to get in the low 30s on gasoline but low 20s on E85 so a high 20s average...ok. But I have only seen high 30s and low 40s when I deliberately drive sensibly...something neither I nor my GF do much of.

 

[Larrymer]

I charted this when I first bought the car. It was within 2 MPG (usually within 1) so I stopped doing it. On my best tank ever (all highway at posted speed) it under predicted, but most times it's slightly over predicting.

Sonic LT 1.4L 6MT (Chevrolet Sonic) | Fuelly

Hover over the notes for my first few fill ups and you'll see the DIC MPG that was recorded. I zeroed it when filling up each time.

 

[mowcowbell]

Mine is over optimistic. Here's a perfect example.

I stopped at the local gas station to fill up with 100% gas 87 octane (100% gas is widely available in OK). My dash computer is showing 33.1mpg for this tank. I have driven 266.3 on this tank. I put in 9.1 gals of gas in my tank.

Hand Calculated: 29.26 mpg
Dash computer: 33.1mpg

Error: 3.83mpg difference

Analysis: Do not trust the dash computer! Heck, my ScanGauge connected to the ODBII port is more accurate than the dash computer.

PS: I reset the dash computer at every fillup, and try to fill up at the same pump at the same gas station (to reduce the chance of pump errors).

 

[ChrisJ]

Even if you use the same pump it could be off by quite a bit, doesn't mean all the error if any is in the dash computer.

Gas pumps are by no means designed or intended to give exact fill ups. They are designed to fill up and not spray out all over the ground. This is why to calculate gas mileage they always tell you to do multiple fill ups and then do an average of those numbers.

Could I see any company making the computer lie slightly? Absolutely.
I also don't know if there is a law effecting it like there is an odometer.