2.73 is the ratio that your gearbox is turning in relation to your back wheel.
If you change your sprockets to 18 48, the ratio will only be slightley different.(2.66)
The advantage to this is, your chain will be traveling at a much slower speed for the same amount of turns of both gearbox and rear wheel.
The chain and sprockets will last you longer and provided you maintain them you are unlikley to have a chain snap and damage your gearbox like they did in the early days on similar gearing set up.
Sam.