Thanks for the replies guys,

well it looks like the only option if i want to use cheap, free parts (my bike cost me $25Aus so the cheaper the better) would be to use the smaller chain ring as the rear cluster, even if compatible has a large cog of 32 odd teeth.... thats almost a compact chain ring!!! im assuming that would bring me massive rear deraileur hassles

so if i was to switch to a 39 tooth small chain ring (which is on the free junk bike) what sort of improvement would i notice? as someone stated before it would be like running a 19.4 tooth cog instead of a 18? so that would make a 13% incline feel like a 12.5%?? lol