Those two tirse sizes will end up being fairly similar. I ran the numbers through a tire size calculator and this is what it came up with:
Specification----Sidewall--Radius---Diameter---Circumference-Revs/Mile---Difference
110/90-16-------3.9in-----11.9in------23.8in------74.8in------------848----------0.0%
120/80-16-------3.8in-----11.8in------23.6in------74.0in------------856----------1.0%
http://www.miata.net/garage/tirecalc.html
Scroll down and use the calculator at the bottom