According to the infamous Miata tire size calulator, your size difference is 1.4% too slow. If the indicated speed is 60MPH, actual speed is 60.9MPH. Not like any of our speedos are accurate anyway. But the indicated speed will be lower than actual speed.

http://www.miata.net/garage/tirecalc.html

It was in the FAQ. Check it out next time.

2000 Contour SE Sport Originator of the Beowulf Headlight Mod and the Beowulf CAI