Mathematics in Motion
Before the widespread adoption of GPS cycling computers, cyclists and velodrome track racers used pure mathematics to determine exactly how fast they were traveling.
Because a bicycle has a direct, hard-linked chain drive (unlike a car's automatic transmission that can slip), there is a flawless, absolute mathematical relationship between how fast your legs are spinning and how fast the rubber is moving across the pavement.
Calculating Ground Velocity
If you know your pedaling cadence, your current gear ratio, and the exact circumference of your tire, you can perfectly calculate your speed down to the decimal point.
The Formula
The calculation simply takes the distance the wheel travels in one revolution, multiplies it by the gear ratio, and multiplies it by how many times you pedal in a minute.
Speed = Cadence * Gear Ratio * Tire Circumference
The Variable of Tire Size
The most critical part of this equation is the Tire Circumference. A standard 700x25c road bike tire has a circumference of roughly 2,105 millimeters. If you upgrade to a thicker, cushier 700x32c gravel tire, the total diameter of the wheel increases. This means that pedaling at the exact same 90 RPM, in the exact same gear, the thicker tire will actually yield a slightly faster top speed because it travels further per revolution!