Here's a Gear/Rpm calculator you can use to see what gear would work well. Go down to the 2nd calculator and input some numbers. It's in MPH but, you can convert to KPH. >>

http://hotrodworks.net/hotrodmath/quarter.html
MPH X 1.609 = KPH

KPH X .621 = MPH

Inch X 25.4 = MM.

MM X .0394 = Inch

Sounds like you have about a 3.73 gear, if your rear tire is 26" Dia.

A 3.08 gear would give you 100KPH at 2500rpm.