I thought it was closer to 8lbs. 16oz=1lb, 32oz=1 quart, 64oz=1/2 gal, 128oz=1gal, give or take a few ounces. Im thinking there's about 120lbs, give or take, in a full tank of gas in our cars, if the tank is 15.7 as advertised. God knows, I need all the reduction I can get, I weigh 320lbs...hahaha.
according to my manual my gas tank is 15.6 gallons. the most i've ever put into my tank is maybe 14 gallons...so i have at least 1 gallon reserve or more. i usually do fill up though at 1/4 tank just cuz it hurts more to see it cost over $30 and there's only a few stations i get gas at.
on teh butt dyno, i always felt my cars were faster after a full tank. maybe cause the extra weight made it squat more and thus changing the angle of my body under acceleration giving a false sense of fastness(like a simulator)