from: Speed versus RPM calculator
the formula relating road speed and engine rpm is
mph = (rpm * cir) / (gear * final * 88)
where rpm = engine rpm
cir = tire cicumference, in feet
gear = gear ratio of your car
final = final drive ratio of your car
88 = combines several conversion factors
for example, the audi 90 quattro (R&T, december 1987). its tires are 195/60VR-14,so our calculated circumference works out to be 72.92 inches or 6.08 feet. its final drive is 4.11:1, its 5th gear is 0.79:1, so their product is 3.25 and the rest is a quick calculation away:
mph = (rpm * 6.08) / (3.25 * 88) = 0.0213 rpm
so, for instance, the audi will do 21.3 mph for every 1000 rpm in 5th gear.
and thanks to google's calculator
1 mile = 1.609344 kilometers
Its fun to sit down and work out your cars max speed (if air resistance and power werent an issue).