I commute on a fixed gear and as I have gotten fitter I have actually dropped my GI from 69 down to my current 60. The key is to spin faster rather than mash harder gears at lower RPM. I can hit the same top speed with the lower GI while putting less pressure on the knees and taking hills faster for an overall faster speed. I'd say don't worry about pushing the big gears until you can average 100 rpm or better in the smaller ones.