Ratio is given by the teeth on the front cog divided by the teeth on the rear one, so for yours it is 20/18, which is about 1.1 The higher the ratio, the harder it is to pedal and the further you go with one rev. of the cranks. Most mods run with 18:12, which is 1.5, quite a lot harder than your current gear. You can get 20t screw on cogs which will fit screwed cranks like the echos, thats the best option unless you're moving the freewheel to the front. (By the way,12t cogs will fit on the same threads as freewheels, but a 12:18 ratio wouldnt be much use at all)