Russ Roth If we assume the cross section of the tire is round such that the increase in width corresponds to an equal increase in height, changing from 2.125" to 1.25", the difference to the radius is 0.875", and a difference to diameter of 1.75", which is approx. 7% of a nominally 26" wheel. To get the same drive ratio you would need to increase the size of the chainring by 7%. 44 x 1.07 = 47, so an increase of three teeth, which would explain
Miele Man feeling the need to swap from 44 to 48.
Miele Man has not told us that his average speed went up when he changed tires, just that he was spinning out, and a smaller tire would definitely require an increase in cadence to achieve the same speed in the same gear.