Cripes the 07' YZ450 pulls like 92 mph bone stock, and with a 45 tooth rear your bike should pull like 115 ? Why you running a 45T rear??
I have a 51T rear as I do a fair amount of woods riding, and the 51T made 1st gear WAY better in the slow, technical stuff
I was looking at changing my rear sprocket, pretty much all we have out here is mountians, would a 51 or 52 be better for steep hill climbs. I find I have to ride my bike way too fast in the tricky stuff.