Seeing how fast you can go in a given gear at a given cadence on this road bike gear speed chart can show you what speeds you are capable of on the high end and low end. Going up a climb you may need a smaller ratio while on a flat section or downhill, you may need a larger gear ratio.
Road Bike Gear Speed Chart
The following speeds are calculated using a wheel size of 700c and a tire diameter of 25mm. If your tire size is slightly smaller, the speeds will be ever so slightly lower while the larger the tire, the speed will be ever so slightly higher.