I think that it is more likely that your gauge isn't reading linearly.

In other words the first "half" of the tank (according to the gauge) is 10 or 11 gallons and the second "half (again, according to the gauge) is 7 or 8 gallons.

BTW, how can you figure that when you've driven 150 miles and used a half a tank you're going to get 600 miles out of that tank? If the other half is the same as the first you'll only get another 150 miles and 150 + 150 = 300 (not 600)

Unless you meant that you drove 150 miles and only used 1/4 tank...

BTW, mine seems to get more miles out of that first 1/4 tank than any of the other three quarters. I seem to get around 140 miles before it gets down to the 3/4 mark - but only around 250 to get down to the 1/2 mark, and 340 to get down to 1/4 tank and by the time I get to around 420 she's gettin' down close to the R (reserve) mark..

__________________
1984 300 Coupe TurboDiesel
Silver blue paint over navy blue interior
2nd owner & 2nd engine in an otherwise
99% original unmolested car
~210k miles on the clock

1986 Ford F250 4x4 Supercab
Charcoal & blue two tone paint over burgundy interior
Banks turbo, DRW, ZF-5 & SMF conversion
152k on the clock - actual mileage unknown