Can anyone tell me what the axle ratio for a MK5 2.0l GL Cortina is?

Cheers,
Mackie.

There should be a little metal tag on the rear cover (it is held on using one of the bolts). Should have ratio on it. If not I would suspect 3.89, but may stand corrected.
Other alternative is to rotate drive shaft and see how many times the wheel spins... not as accurate but should give you a good idea.