well, wheel RPM at 6000 engine RPM would be 6000/5.5, or about 1090 RPM.
Multiply that by 60 to get 65454 revs per hour.
Multiply by pi and 20 to get 4.1 million inches per hour.
divide by 12 to get 342000 feet per hour,
and divide by 5280 to get 64 miles per hour.
The only problem is that 2.5 HP isn't going to be able to overcome the wind resistance to push someone along at over 60 miles per hour. So, the engine will bog down, and never get up to speed.
I would recommend adding another 2:1 or so in reduction. Which would drop the max speed in half, but you could achieve it. You don't say what kind of 2.5 HP engine you have, but the honda GX50 engines have a top RPM a little over 7000. Ading 2:1 additional resuction with a honda GX50 and existing 5.5 reduction, and you could top out at about 38 MPH.
problem i got is this was to be a starter project so im having trouble finding the gears to make the ratios. i was gonna basically have a final drive ratio from the clutch to the wheel using minimoto components. any ideas?