Sorry i did not get back to you sooner. I use 17/23 60/56. I too was running out of gear, but not at SpeedWorld. I ran a 55 spur at 301 which has a 250 ft straight and it needed a bit more gear. I've heard there is a 24 tooth pinion out there, but I can't find it.
Sholman- Thanks for the sirio clutch setup info.. I tried it on the mugen this weekend and had some success with it. Quick question would you happen to remember what gearing you ran at Speedworld for the Mugen Challenge? I was running out of gear half way down the straight the car would completely stop pulling. Not certain this kyosho/sirio setup is best for weekend racing, maybe for larger events where the traction is up a little bit.