Trad Morgan's rely on the tyres as part of the suspension so reducing the depth of sidewalls gives a harsher ride but possibly sharper steering. Obviously increasing rolling radius of wheel and tyre combination increases ground clearance but will also increase gearing reducing acceleration through gears and speedometer will no longer be accurate, although a small increase may improve it
With the lighter 4/4's narrower higher profile tyres as factory fitted seem to suit best so any changes should be of a minor nature dimension wise.
Later Roadsters came fitted with up to 205/55/16 tyres to better accomodate the power on the rear at least, but most had electric steering assistance.
You will get more specific advice if you advise what model you have with what wheel/tyre combination and what you wish to achieve, i.e. better ground clearance, handling, comfort.
