RPM of the rim remain the same regardless of where you place the magnet on the spoke. One turn of the wheel at the tire is equal to one turn at the spindle.

Magnet sensor read each individual pulse when the magnet passes the sensor. Since your bike will move X amount of feet with one revolution of the wheel, the sensor will see only one pulse for that one revolution regardless of the magnet is closer to the outside or inside of the wheel.

Edit: forgot to include a formula:

(rev/min) * (ft/rev) = ft/min

then:
(ft/min) * (mile/5280ft) * (60min/hour) = miles/hour