I've only got back into cycling proper in the past 12 months, but I've really caught the bug. I bought an entry level Raleigh Airlite 100 a year ago and was initially really pleased. But as my fitness got better I've not been able to keep up with my training buddy on flat/downhill sections (despite beng faster climbing), as my gearing just won't allow me to keep up. This bike has a Chainset Prowheel Chariot 320 52t/42t - 56cm=170mm, 58cm=175m. I don't really understand what the latter cm/mm mean (sorry for being such a novice!) and I'm looking for advice on how best to tackle my inbility to crank up the gears and really power on the flat and downhill...?

For the record I'm averaging just over 20mph on rides of an hour or so and feel I could raise that if the bike would let me. What should I do, without it costing a small fortune?