On my Tanaka PF-4000 GEBE with a 13T pulley top speed maxed out at 28mph. The engine wasn't broken in yet. The GEBE i have now only has a 12T pulley. It is matched up to a Subaru EH035. Not sure what the top speed will be, but it will be quieter.
The sooby should be fine with a 12T, but that was what the raleigh had when I bought
it. The belt was shredded & the pulley badly worn. I won't be getting any slippage with
the 15T. the PF4000 has considerably more torque.
The Raleigh pedals great without power, something I can't say for my electric or
GEBE Giant mtn. bike. Ready to travel below, (stealth mode engine cover), in
addition to the stock 1 liter tank, the 3 liter jerry can gives me a powered range of
160mi.