There are 11.9 gallons to our first gen's gas tanks. If you put in 10.5 gallons regularly, then you'll have 1.4 gallons left. If you average 30 mpg, for example, you'll have ~44 miles left to drive.
And you're correct that the final 1/4 tank appears to burn quicker as per the gauge, but it's just an illusion. Remember that for you, the red light comes on with 1.4 gallons left, so it isn't a fair comparison.