I filled the tank yesterday and screen said 440 miles to empty I've driver 12 miles and now it say 385 miles to empty, what gives?

 

Seriously? This is your emergency that requires multiple !!!! ? Chillax. Trip computers adjust constantly taking into account how much fuel has been used and how far you have driven since the fill up. If the 12 miles you drove were on the highway then the computer calculates the miles remaining based on those miles, but if it was stop and go city then it is based on those miles. Personally I pay exactly zero attention to the miles to empty and focus on maintaining my average mpg's which helps me drive with a lighter foot.

 

As stated above, you are better to relate to the average mpg.

I have a Ultragauge fitted on my V40 which shows average litres/100 k's,equivalent in mpg's and also gives you instantaneous readings as well.
The instantaneous readings vary a lot and react swiftly to the slightest change in throttle movements.
Overall they are good to monitor as you see the affects of using too much lead foot. LOL

 

when that reading says zero it means zero so why did it take off so many miles? I drove it on the hwy with a light foot, could it be the ac? the first tank did real well.

 

It changed so much because it completly recalculated the number based off of the info it gathered while you were driving.

Did you reset the computer before filling up and seeing the 440 miles? The computer averages out all of the data stored in memory so I wouldn't trust that number (or at least trust it less) until you have a few hundred miles of data in there.

I think my S40 says something close to 380 miles after a fill up, but I rarely look at it anymore. I don't start watching the gauge until it gets below the 1/4 tank mark.. then its time to find a place to fill up again.

Also I prefer to just reset the main trip after I fill up and look at the miles once its close to empty. ~10 gallons in the tank, and these cars usually get around 26-28 average... so after 260 miles you should be very close to empty.

 

Keep in mind that what you are getting is a mathematical calculation based on numerous sensor inputs, not actual fuel use, so the accuracy is very questionable. After monitoring trip computers from multiple cars over the last 25+ years I can tell you that best case they are +/- 10% and no better. You put this thing under a microscope for 12 miles and the shorter the monitoring period the wilder the swing possible, but eventually it will come back into a more normal range. Also, believe it or not, zero isn't usually really zero. I can't get more than 18.5 gallons in my 21 gallon tank even after the computer shows zero miles left. It really hasn't bothered me enough to figure out exactly how much further I can go on zero miles, but I'll bet it is at least 50 miles. You just can't take these things literally or expect them to be precise.

 

Thanks for the info guys this thing was starting to drive me ape **** crazy