IMHO wider tyres serve no purpose on the Plus 4. It is quite probable that you will get reduced grip on dirty or wet surfaces, steering will probably be heavier (that is why so many Roadster owners opted for power assisted steering. Also it is possible that the tyres could be outside the mudguards. Is that an MOT failure in the UK? I doubt mudguard extensions are available for the Morgan laugh

I have fitted 195/65 15 tyres to get better ground clearance. if you want better grip just get the correct tyre size in a good sports compound.

Regarding Speedo calibration you may find no change is required because the standard setup under reads the speed and distance. You can calibrate the speedo after travelling an accurately signed distance (say 10 miles) and compare that to the odometer distance. Calibration is not difficult and instructions can be found on the Smith's website

Peter

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