Thats weird, according to gearing commander.com with 16/44 and a 50 series tire it should be able to do 149 @ 12750 in 5th and 160.7 in 6th. 16/42 156 for 5th and 168 for 6th. Now if it were 15/44 it'd be 140 and 150, 15/42 147 and 157.8. Are you sure it's 5th, 6th and not 4th, 5th or you actually have 15t on the front. Not to say you don't know what you're talkin about, just fresh thinking to the table. I'm runnin 15/42 and just gettin into 6th at the traps @ 144. My bike stock is 17/40. Now also according to gearingcommander 5th should top out @ 147 for me. So maybe those mph are a little (roughly 3 mph). I just have a hard time thinkin they're 10 mph off. BTW Are you sure you're not hitting the speed limiter with the gauge reading 186? Just some ideas...
'06 "Corn Burner" Edition ZX10R
w/ a lot of bolt-ons
PERFORMANCE CYCLE 2 Racing