Just posting for FYI:
I got my FDR down to 3.694 and that seems the sweet spot for me at the moment.
I have 37t Diffs, 19t lay shaft gears. I am running a 74t Spur and a 39t Pinion
running 17.5 Hobbywing motor, Justock esc. I do have a fan on my motor
Outside temp 70 degrees, Track Temp 85. After about 5 mins motor was at 145. after 10 mins it was 165. This gives me some room to play with once the weather gets a little hotter.