Use an HRM and see for yourself.
It depends on how hard you ride... I'll burn close to a thousand calories an hour if I time trial it to work, but my calories per hour drop significantly if I take it easy (to around 600/hour)- but of course, it means more time on the bike, so the same distance may burn around 800 for the 20 miles.
Your gearing will affect your calories, as do hills, wind, etc... so there is no real formula. I'd argue that a geared bike allows you to maintain a target heart rate more easily.