Weight ⇄ volume (avgas). Avgas weighs about 6 lb per gallon.
Pounds = gallons × 6  ·  Gallons = pounds ÷ 6.

Fuel burn. Fuel used = burn rate × time (burn rate in gallons-per-hour, GPH).
• Used = GPH × hours   • Endurance = fuel ÷ GPH   • Burn rate = fuel ÷ hours

Always keep a reserve. Plan to land with fuel to spare — this game uses a 30-minute reserve. Fuel needed = GPH × (trip time + ½ hour).