Steve and Duane
Thanks very much to both of you for your comments. You seem to disagree about the speed of Arduino. Being quite new to this I would have thought that 1800 RPM is relatively slow. The idea of a running average is a good one. I will need to get my head around how to code it.
Duane, your work with your lap timer looks really interesting. My problem is that at the venues I compete at it will not be easy to place beacons around the course. The organisers use an infra red timing system at the start and finish. Your idea of a buzzer sounds good. My objective is the same, to get immediate feedback on the part of the course I have just driven. I will spend some time looking at your work and get some ideas. You are at a much more advanced stage than me.
One other thought I have had is to build a test rig using a magnet mounted on an electric drill and use serial print to analyse what exactly is being missed at higher speeds.
Steve, I take your comment about looking at lights and displays while racing. I already have a shift light which is easy to see in my peripheral vision, I don't think another couple of LED's would be a problem. The display might be.
Thanks again both of you for your help. I haven't given up and will report how I get on here.
Graham