Join Date: Feb 2012
Location: Colorado Springs, CO
If you've got that many miles on your stock chain and sprocket you're definitely going to want to change both front, rear, and chain. They create a wear pattern when they run together, and putting a new sprocket, or just one sprocket and chain can screw up everything (including the new stuff).
And yeah 80 MPG isn't unreasonable if you're riding carefully and slowly.