Ferret,
It depends on the TYPE of donut. If it is a Krispy Kreme then the high sugar content dictates only two. However if it is a Donut King then 3 donuts are about right - although as Commuter and the other "downhill specialists" will attest, the carbo-loading item of choice is a walnut scroll from Bakers Delight.
Will you be racing at Calga today?