[Blue Bomber Man]

I think everything Ive posted above clearly answers your question about tires size and efficiency. RRC is way more important than size (and size's effect on gearing). Generally speaking 15-16" tires have the best average RRC, however the lowest one I could find researching was mentioned earlier, it was a 14" tire. Now if you could find a 15-16 tire that only had slightly worse RRC, then perhaps the 15" tire would be worth it for the gearing benefit.