OK,

trying to do some gear-inch/rpm math and the numbers aren't coming out right.

I'm imagining something that should be really fast and has nice even numbers, i.e. spinning a 90 inch gear at 120 revs/minute

how many miles/hour is that?

so:

90 inches/rev ÷ 12 inches/foot = 7.5 feet/rev

7.5 feet/rev X 120 rev/min = 900 feet/min

900 feet/min X 60 min/hour = 54,000 feet/hour

54,000 feet/hour ÷ 5,280 feet/mile = 10.227 miles/hour

intuitively i feel that this answer is off by a factor of at least three, what's wrong with this math? Is my comprehension of gear inches off, or am I just doing something really stupid?