your going up hill in a big truck ,18 wheeler at a (*10 % Grade) or (* 5.7106 DEGREES) your fully loaded at (*80,000 lbs) or (*36287kg) or (*40 short tons) your traveling at (*30 mph) or (*13.41120m/s) you travel for about (*10 miles) or (*1609.344 Meters) the actual distance traveled would be 9.9499 miles Based on real world data you would use about 2.5 gallons of diesel fuel. That is (4mpg: miles per gallon) companies whould have you belive this number was higher For this exercise lets say 3 gal of fuel. thats about (3.33mpg) now here is the senerio 1) you get to the top of this hill and stop on a bridge that is old and about to fall. The bottom just happens to be the exact height that you started. 1 mile down. your Gravitational Potential Energy in Jouels PE=Mass (kg)*G(9.8m/s)*H(m) PE would be 572,303,004.13 or about 572 mega jouels or fairly close now a gallon of fuel has a potential of anywhere from 45.3 to 141 mega jouels of energy based on where you look my pysics text book gave 45.3 while a web search turned up many numbers in that range, all different. so we will use the largest of 141 141 * 3 (actual gallons used) = 423 mega jouels of energy to get a potential of 572 mega jouels Where did I go wrong?