Is there a chart that takes gear inches, multiplies by stroke count and converts to MPH?

I just bought a fixie/SS that has a 48x18 drivetrain. That equates to 72". If I multiply 72 times strokes per minute that gives me inches per minute. Divide that by 12 to make it feet per minute and multiply by 60 to get feet per hour. I then divide by 5280 and that should give me miles per hour. At 80 rpm, my math says 5.45 mph. That can't be right. I know it's got to be faster than that. Where is my math error?

For example:

72" x 80 rpm= 5760" per minute

5760"/12"=480' per minute

480' x 60 minutes=28800' per hour

28,800'/5280'=5.4545 mph

At 120 rpm it comes to 8.18 mph. To average 15 mph I need to cranking at 220 rpm? I must be missing something. Would someone please educate me.