I'm a reasonably fit guy...I think and i do all sorts of cycling; mountain, road, recumbent. But I've been training harder in the last year and I can now cruise on my road bike at 30kmph fairly easily for an hour or more (dependent on conditions). The problem is that is the fastest I can ride without having my legs spinning around franticly. Part of my problem is that my bike is old (25yrs+) and i cant afford a new one.
I think Im like most cyclists. I always want to improve my riding times and distances but the way my gears are set at the moment it just isnt possible.

So what Im considering is changing the rear cluster. Its currently a 30-14 cluster with a 58 tooth chainring. How much lower can the cluster go than that and how much would it cost to get a new one?
I could quite easily do it myself but before i do i would like to know what would be best to get.

Thanks guys