Some takeoff weights from the E6B page:
Fuel Guns Ammo Fuel Weight
25 8 425 92 13282
50 8 425 186 13849
75 8 425 278 14339
100 8 425 370 14951
From 25->50% fuel, you gain 567lbs and 94 gallons of fuel
From 50->75% fuel, you gain 490lbs and 92 gallons of fuel
From 75->100% fuel, you gain 612lbs and 92 gallons of fuel
Shouldn't each 1/4 of a tank of fuel weigh the same amount and have the same number of gallons?