http://www.miata.net/garage/tirecalc.html
Put in your standard size wheels, then put in what you want to swap it to, and it`ll tell you at the bottom what the overall percentage difference is.
If you are putting in a modern type TDI motor, you`ll need to sort the gearbox ratios etc out to make the most of the changed power and torque, modern motors make all their torque at a lot lower revs than the old diesel motors, which were more like a petrol engine as far as revs go. My T3 is doing 3000 revs at 60 mph, a modern diesel won`t be happy at that, a modern diesel car will be doing 2000 revs or less at 60mph, max torque is usually starting under 2000 revs, which is 40mph in a T3

Keep it real.
Search first - ask second ;>}