Roughly, it's "rise" over "run"... not 100% accurate but more than close enough.
"Rise" would be your highest point minus your lowest point.
Divided by
"Run" which is how far you rode in going from the lowest point to the highest point.
edit: for your example, I came up with about 3% ( 80' / (5280' / 2) )
Last edited by SaiKaiTai; 08-10-07 at 04:43 PM.