That issue came up previously, at least for those that have the nav system - and I think it has to do with the fact that the computer only averages out 2000 data points when calculating the average MPG - and this may be well under one gas tank. For those that have the in-dash economy display, I'm not sure if this is the same situation, but could explain why the reading is off. So instead of averaging the whole tank it only averages the last 2000 readings. Someone asked this question at the Dearborn meeting - but what wasn't clear how often those data points are collected. I think it may be every time the instant economy reading is updated.. maybe once every couple of seconds or so.

I'd be interested to know how the "miles to empty" is calculated. I filled up recently and it told me "478 miles to empty." After driving around for about 20 miles and making a one hour stop later that day I got back in the car and it said "490 miles to empty." Wonder how often that gets recalculated.