I'd like to know gearing for 13.5 as well. That should cover gearing ranges for 2 motors most use. thx all for the help!
I replied to your original question a couple months ago...
http://www.rctech.net/forum/14555867-post11075.html
...but have since changed my gearing to something more suitable for my particular 13.5T and track. I'm now running 27/81T for a 7.5:1 FDR. Some of the guys locally are running up to a 8:1 FDR. I'd play around with gearing anywhere from 7:1 to 8.5:1 FDR to see what suites you and your track the best.