To link all the available flux of a magnet you need turns bigger than the magnet. If you have rectangular magnets, any turns of a coil bigger than the magnet will link all the flux. If the inner hole of a rectangular coil is the size of the magnet you use all available flux.
Any smaller coil or using a round coil with rectangular magnets will mean that at no time will you link all the m,agnet flux so from a purely voltage point of view you will loose out but that is not the whole story, the shorter turns will have less wire and less resistance so although you have less than the ideal voltage you may still have a more favourable volts/resistance ratio.
It really is not too easy to guess how these things interact but it seems from results obtained that you can bend the rules a lot.
Using round coils in your case almost certainly does equate to the same thing as opening the gap but the reduction in resistance could possibly mean that you may need to add series resistance if you can't get the thing out of stall after increasing the air gap to the point where your cut in speed is in the 7 to 8 mph region.
Flux
