The new kit wheels tend to tear up the balls, according to a couple of reports from teams who tried them. We're using a single 8" kit wheel from a year or two ago, driven by last year's FP motor with it's 19 tooth pinion, which drives an older FP plastic first stage gear from the FP gearbox. The ratio is 19:72, I believe. This gives us a pretty good range....
http://www.youtube.com/watch?v=81vOT2KY2ig
But the gear ratio you want will depend on a lot of factors, as others have mentioned.