No, that should work regardless. He wants cadence. If he puts the magnet and sensor on the crank, it'll read a variation on cadence regardless of how fast the bike is going.

Assuming it's set to KPH. The wheel diameter setting is in millimeters on every computer I've seen. He wants a per minute reading. The comp is trying to deliver a per hour reading. So what you want is a setting that returns 70 kph when he's turning 70 RPM.

To make it think you're going 70 KPH at 70 RPM, obviously you have to make it think that it's attached to a tire that will go 70 KM when it turns over 70 * 60=4200 times. That means a wheel that's 16.667 meters in circumference. OK, that's not going to work, no bike comp can take 16,667 as a circumference.

So divide by 10, put in 1667 as the wheel circumference (this is very close to a 20" tire). It'll say 7.0 KPH when you're going 70 RPM.

This is actually a very clever thought by CaptainCool. It means you can pick up the crappiest $8 computer at Wal*Mart and use it as a cadence readout.