For a 1 foot diameter rotor the circumference will be 3.14 feet so in a windspeed of 10mph, the outer edge of the rotor will be going 52800 feet/hour or 88 feet/minute.

Dividing the circumferencial speed by the actual circumference gives you 88/3.14=28rpm

I believe optimum loading for a savonius is when it's running at a tsr of 1/3 so to extract maximum power you'd want your alternator to hold it down to a little over 9rpm in a 10mph wind.