46-15 or 46-16 for my fast bike. 39-17 for my screwing around bike. I like spinning downhill, anyone running above 90 for urban riding is either a veteran rider with massive legs or looks very silly taking off from a stop or climbing. I have heard that no matter what ratio you are trying for it is better to use the largest possible ring and cog combination to achieve it. This apparently makes a lot less stress on the whole drivetrain.