I don't know if anyone remembers me asking a few months ago, but when I first got my car last August my DIC was accurate as far as the gas mileage. Then after a few weeks it all of a sudden became inaccurate. I would fill up the tank, and the DIC would be between .8 and 1 full gallon low on how much fuel was used. Meaning, the DIC would show I used 12 gallons, but I would actually put in 13. So accordingly I calculated mileage about a full MPG less than the DIC showed. Well I had this happen every time I got gas until Thursday. I filled up, and low and behold I put in the same amount of gas as what the DIC said I used. And so far my mileage on this tank is higher, so I'm guessing the same will happen when I fill up in a couple days.

Does anyone know if this could be related to the change between summer and winter gas? My other thought is if the station is fiddling with thier pumps to make some extra cash in the winter, but that seems unlikely.

I'd need to look into how the car calculates it. Whether it'* injector pulse, actual tank reading...it'* using something to try and determine fuel used per mile.

Did the fuel gauge look to be any higher or lower than actual fuel used during these periods?

We need to know what the specific gravity of the winter and summer blends are. That may be a factor, but may also be very hard to find.

I'm not sure if the guage looked any different. I don't drive the car much other than to work, so every week is very much the same. I don't remember exactly when the switch happened, I'm thinking maybe October. I will fill up after about 320 miles each week, and put in 13-14 gallons. Every week, without fail, I will put in between .8 and 1 gallon more than the DIC said I used. The DIC also would show between 24 and 24.5 MPG every week, but the actual MPG was 23-23.5. Well when I filled up Thursday, I put in 13.3 gallons, and the DIC said I used 13.2. And the MPG on the DIC was 25.0, and I calculated 24.9. I also always get gas at the same station every week. It will be interesting to see if this happens again in the fall. If so I'll have to get gas somewhere else and see if the same thing happens.