I'm confused by, uh... your confusion. I did a test fit of various 64P gears on my TC6 and can fit anything from 112/22 (FDR = 10.18) down to 76/60 (FDR = 2.53) without modifying anything. For USVTA with the 25.5 motor (assuming your local track uses full USVTA rules), you'll probably want to gear in the 3.5 - 4.0 range depending on the track size. There is NO limit to FDR in the national USVTA rules.