i ride a bianchi conversion fixed gear with a 46/16 gear ratio. i've found lately that the only thing that ratio is good for is climbing hills. i want to buy a new rear cog that would give me a higher ratio. i live in west michigian and it is full of hills. i want a ratio that's good for flying down steep hills because 46/16 is awful for that. i want something high so i can go fast on flatland and downhills. what's a ratio that is good for that while keeping the 46 tooth chainring?