You can do leg speed work on rollers or a turbo but its not as good as doing it on a track, or coming off of a big hill onto a long flat. Preferably where there is no risk of hitting a car. Anyone who knows me well is laughing right now......
72" is the kind of gear you would run if your riding your fixie around to get into shape for race season. If this is what you wanna do then you have the right idea. If your trying to train your final attack or flat out sprint leg speed than you need a differant approach.
82 is plenty light, "lighter gears" in a track riders frame of mind is in the mid 80s. In any sence you need to pick a gear thats going to allow you to make your legs go faster than they want to. Witch means you have to try to stay on top of the gear. Do one of the following a few nights a week.
Turbo/rollers. Warm up for 15 - 20 min X5 10 second rev ups to max cadence, 5 minuits or more recov.
Hill sprint. Find a road with a steep enough hil on it that you can roll up to a good speed, then sprint 50 meters to the bottom, and spin your ass off for 200m on flat road. IF you have to ride 25 min to get to a spot to do it perfect! do 4 of those with plenty recov time. 15-20 minuits. Pick a gear according to the hill, you may find as you do these your able to move up a couple inches. Please watch for drivways.....
On the track, flying 200's chasing a much faster rider on a bigger gear than you. X4 with plenty rest. Even if you get dropped by 100m keep going.
Most people when they start doing over speed work forget the principal. The idea is to smooth out your pedal stroke not continue to bounce on your saddle. If you have too, lower your saddle a touch.
Its amazing how much speed you can gain doing these. When I started working with my coach I didnt have enough time to spend a block in the gym to get any stronger so he just had me make my legs faster for the strengh I already had. Within 3 months I was going 4-5Km faster on the same gear leading into nationals. If you teach your legs early how to go that fast, it only takes one or two efforts a week to keep that. Most pro sprinters do an over speeding effort after their initial warm up to fire up the legs even more before they go up in gearing.