The TCH gauge is set to keep around 2 to 3 gallons in reserve. My guestimate is the low fuel light = 3 gallons, and Empty = 2.5 gallons left in the tank. I would say for normal driving that 1/2 tank is pretty darn close to half a tank, leaving close to 2 gallons in reserve.

I often see 50 @ Full, 180 @ 3/4, 340 @ 1/2, 540 @ 3/4 and 640 @ Empty. The low fuel light usually comes on about 600 miles. I usually fillup around 660 to 680 miles ( 20 to 40 miles past E - filling up with around 15 gallons ) Keep in mind I typically get around 44 mpg with my TCH. Your mileage may vary

I find the MFD displayed Tank Average to be about 1 to 2 mpg higher than my actual calculated mileage when I fill up.