Originally Posted by DannoXYZ

Here's a thing about biking that you may find if you search around the training or racing forums. There is a criss-cross function between the cardiovascular system and muscular. Maximum performance can be achieved by balancing those two factors at their maximum output. Meaning 8-miles in around 20 minutes.

The general rule-of thumb in selecting proper gearing is:

1. if your legs hurt (lactic acid) but your lung/heart isn't working hard (low HR), then shift down to easier gears and spin more.

2. if your lung/heart is hurting (high HR), but your legs don't feel a thing, shift up into bigger gears and push harder.

Basically you're using too large of gears and pushing to hard. Shift into an easier gear and spin more, actually shift down two gears at the same speed and you should be in the ballpark of the correct gear. Your legs will hurt less, your HR will be higher and you'll be able to go generate higher power continuously and go faster, farther and longer.