Machinehead is useful for calculating the speed vs cadence with his ratio sums,
but can you keep the revs in the long gears ????
You need to get on a turbo with power measurements (like Redbike's Tacx magic ) to see how much oomph you can muster, and how much oomph you can keep for an hour.
Then you need to estimate your coefficient of drag by freewheeling down a hill.
Then you can marry them all together and choose the gears that suit you.
There's another thread that talks about 50T rings and 11T sprockets. Is anyone who isn't a pro' able to keep 85 rpm for an hour in this gear? 30 miles !!
Probably not, its for downhilling ( or posing ).
If you can keep 200ish Watts for an hour, you could cruise along at 20 mph on a 53/17.
I ramble,,,,