Relationship between speed and cadence? I am having a disagreement with a friend.  It stems around the scenario of 2 identical bikes on the same gear ratio side by side.  If the rate of pedaling is same for both do both bikes travel at the same speed?  Or does the rider's weight play a factor?
Speed is determined only by gearing, of which crank, cassette, wheel, and tire are components

Any 2 bikes using, say, 42t cranks with a rear 32t cog and 25c tires on 700c wheels at 90rpm will be going the same speed. Bike type and size, rider/bike weight, and even front wheel/tire size and crank length don’t matter. These other factors only effect how much power it will take to keep up that cadence with that gear combination.