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@gbj_tester
I use a spreadsheet (attached), but the formula is
Speed/mph = (Chainring/Sprocket) × wheel diameter/mm × cadence/rpm × 1.171e-4
The units are the common ones, which is why the conversion factor is a funny number (including the pi).
The sheet is approximately correct for 700C tyres not too much wider than the rims they are on, it doesn't do a fully rigorous calculation of tyre rolling diameter from cross section and rim width, and it takes no account of tyre drop under load because gear roll out is measured with no load for MG gear check, the only time when that degree of precision is of interest.
The science nut in my house wants the units to be m/s, m and rad/s, in which case
v = gear ratio × wheel radius × crank angular velocity
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