WhiskeyHotel

Here's how to calculate it:

mph = (rpm * cir) / (gear * final * 88)
mph = (3000*6.96)/(0.794*3.692*88)
mph = 80.9

where:
rpm = engine rpm (We'll use 3000)
cir = tire cicumference, in feet (We'll use 83.5" or 6.96' for 19's)
gear = gear ratio of your car (Assuming an MT in 6th, this is 0.794)
final = final drive ratio of your car (Assuming an MT, this is 3.692)
88 = combines several conversion factors


To work it backwards, using 73 MPH:

RPM = (mph * gear * final * 88) / circumference
RPM = (73*0.794*3.692*88)/6.96
RPM = 2706

This should get you close!