70-85% of your maximum speed

Whenever you are running relativly fast or kind of fast, you are doing a tempo run, however not good enough to do always and just improve. Soccer players, when running around the field run like this a lot, but do they improve their technique or form? Nope.

The only way for a tempo run to help a sprinter, is by doing it while going with good body form and near perfect technique. Arms relaxed and LOOSE AND NOT TIGHT, driving your elbows back as your hands come to your hips and back up to near the front of your face, done all at a very relaxed position. Upper body leaned very slightly foreward, relaxed muscles and staying straight up and not moving body around. Your ankle (bottom of it) must step over your knee as your knee comes high and as the foot on the ground is lifting and pushing off, and your legs must take LARGE STRIDES while maintaing your high knee lift and not kicking legs too far back to your buttocks. Remember to keep your arms and legs in balance and co-ordinated!

What this does is it improves endurance in general (aerobic capacity and aenarobic lactace system). However it teaches your neuromuscular system by making your neuropathways (brain to muscle) learn the proper body and leg movement needed for better speed and stride. So your teaching your body to run properly. By doing tempo runs AFTER drills, your body understands the drills that you do much better.

For better conditioned athletes, going farther is probably better as long as you can maintain your PERFECT form and technique. Practice being loose, while keeping shoulders in place and relaxed, while driving arms and keeping body straight up. Also do drills to teach your legs proper driving and knee lift. Spend much time on drills so you can get perfect tempo runs in order to teach your body what it needs.