Just went out to test. At 6,0000 rpm it's doing just over 40mph. It's not that noisy in neutral, just a bit of a rattle. 5th gear seems no longer than 4th from what I can tell. Its in a 2005 Birkin rather than Caterham
That's what I feared.
I was kind of hoping someone would say they might be straight cut gear, but the whine in neutral, and no whine in 4th, both suggest its more than that