I did the difference once and it was with a 8inch rim and 235-40-17 and a 235-45-17 tires and the diff was not enough to have an effect on the speedo.. Here is the tool I use to figure this stuff out.. I went with the 235-45-17 just to have that extra bit of sidewall..
Its from the Miata guys, they swap tires more than we change our socks..
Tire size calculator
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The silver 03 GT is gone, but a Solstice has arrived. There are three more PT's to play with though
My personal thanks to veteran and novice crewbies that have offered information and how-to's they have been instrumental in taking LiquidPT from modded car to stock to live a more quiet and sedate life.