Well, I think that you could do a few mods at one time, and fix all the problems that you may cause with the different tire size.
1.) It couldn't possibly THAT much of a size difference that it will throw off racking up an extra mile for every 10 you drive. I doubt it could even add one miles for every 100. If anything, you are talking an inch of difference in circumference. Maybe 0.7". Who knows.
There is a good site to get an approximate size difference [http://www.miata.net/garage/tirecalc.html]
. Note that how wide the wheel is, the tire manufacturer and a few other variables will affect the total final tire circumference. That site is a great starting point, though.
2.) To fix any problems, AND gain some performance -- purchase an X2 Power Programmer from Bob over at Jusnes Modified and he can re-program your car with more power, better torque, increased throttle response and fix the odometer and speedometer with the different tires and wheels.
I already have a tuner, so when I get tires and wheels, I won't have to worry about the size. he can send me the tune and I can load it up!
Just for fun, today I looked up some 19" wheels vs. 20" on the pricing issue. The 19" are a bit more money -- but I just feel that for MY needs -- 20" is too big, 18" is not worth upgrading from my stock 17", and 19" would be perfect and not commonly seen...
I guess being unique and perfect as to what you need takes money...ugh