Forget the empeg get a cycle computer

Strip the magnet out of the bit that fits to wheel spoke and stick it inside your wheel. then mount the sensor somewhere so it can pick up on the magnet.

Then extend the cable which is a bit tricky then mount the cycle computer in the car set the wheel diameter and there you go

You running 18,s should cause no problems as this is a common thing in caterhams and we run 13's.

the other solution is to get a small gps with serial out and it should be able to squirt speed out down a cable to the empeg.

Or use my solution borrow a GPS and go out and work out what revs in what gears are what speeds. the you just use the rev counter.