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The MX5 has what we call zero scrub radius, consequently there is little feel through the steering wheel. Most cars have something like 30 mm scrub radius. We have all heard cars squealing in car parks. This is tyre scrub.
Exocet looks great with a wider track width than the MX5, of course you cannot widen the MX5 without cutting out the wings on Exocet you can. Wider will give a better stance and better cornering.
The ET of your wheels (also known as off-set) is going to determine the track width.
Basically you should aim for a zero off-set.
Example: If ET 38 is stamped in the back of your wheels then you should get 38mm wheel spacers.
30mm spacers will be fine though, we do not have to be accurate to within a few mm.