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The MX5 instrument binnacle including the speedo can be transferred directly from the donor to the Exocet and simply plugged back in. If you put the old MX5 wheels back on for the test your speedo will be calibrated to those. It may spoil the looks but not the test.
The 15" wire wheels/tyres I used on our demo car were just within the limit of accuracy for the IVA. If you need to adjust the speedo to suit your wheels you can always pull the needle off the speedo and move it around so that at standstill it reads say 15 mph, this does not result in a fail of the IVA. I like to see a speed of around 10% faster than I'm actually travelling at.