Mine always comes on exactly at 50 miles remaining...
In my experience, it appears that the miles remaining estimate is based on recent driving. I have noted the following example before, although I'm sure the numbers are slightly off.
Miles remaining reads 75 miles. Approach freeway on ramp and WOT 3rd gear practically redlining it to get up to speed (85 mph speed limit so don't get all preachy). Shift to 6th and note that miles remaining reads 62 after driving less than a mile. Cruise in 6th gear for a few miles and watch it increase to above 75 miles remaining
