I know of no formula that can make that determination. One would think the larger displacement would have the advantage, but there are more variables than that. The only sure fire way to tell is to run them and see what they will do. If the 60 is capable of swinging a bigger prop, then it is capable of swinging one with more pitch, which will increase the top speed. I like to run 11-7 on 60's and 10-7's on 46's. If your 60 is a hot one, maybe you can run an 11-8 or a 10-10.