Re: top speed?
just to be a voice of dissention here, a 45 degree hill would be a 100% grade.
the steepest street in the world is Baldwin Street, in a suburban part of New Zealand's southern city of Dunedin, at 19 degrees, or 35%.
Superior Hill, in Newport Beach, California, is a 6% grade, which is about 3.5 degrees. the reason i know this, is i got a ticket for riding my skateboard down it in the 80's. there's a law in california saying you can't skate down anything steeper than 6%.
my first bike, a 26" with a 36 tooth sprocket, would clear the hill without pedaling, but just barely. like 5mph or so at the top.
now, if you had a very short ramp at 45 degrees, you might go over it at 15mph. or, you might stop abruptly and flip over the bars.
in case you're interested, wheelchair ramps are usually 4 degrees or less...