The gearing is irrelevant to whether it's fixed or not.
The ability to coast is over rated, massively over rated - where you may coast on a bike with a freewheel, it's easy to just let your legs roll over and it's better for them (keeps the blood flowing to flush out the lactic acid)
I ride 66 gear inches after finding that 70 felt a little high at times. On the other hand, I'm a spinner when I a geared bike too.
For the record, my average speeds and max speed are the same over the same route whether I'm on my fixed gear or my geared bike.

Gearing. It's a simple matter of making a guess, trying it and modifying to suit.