I chose a gear combo with the same gear inches and a higher gear ratio. I chose a 46 x 15 for this bike, which is the same as 46 x 17 with 700c wheels and the same width tires. The bsd (bead seat diameter) of a 650c wheel is 571mm, whereas it is 662mm for a 700c wheel. So gearing increases about 16% when you go from 650c to 700c, but it is slightly less when you take the ratio of the actual outer tire diameter, more like 14%. Anyway, to get the same gear inches with a 650c wheel as a 700c tire, it turns out pretty close to reducing the rear cog tooth count by two.