Speedo issues.

 

Yes, I realize that, but I'm not too concerned. If anything I can use the gps speedo on my phone to figure out how off it is.

 

Hopefully there wouldn't be an issue in rubbing/bumping. not a HUGE difference, but it's about 15% taller sidewall, and on the inside of the front tires it doesn't seem like there's a whole lot of extra space. Maybe call Tire Rack and ask them about the size. They've had the ST in for tire fitment. It's definitely not a recommended size, but maybe you want that extra sidewall for the extra cushion/pot-hole protection during the winter ...

 

Here is the link to the famous Miata Tire Size Calculator. I've been using this for years. Usually this will recommend that you don't exceed a 2% diff in speedo error.

Tire size calculator

 

Well, it turns out they fit, as long as the car isn't moving. The front was too close to the fender liner for my liking, so I passed. It was worth finding out though.

 

definitely!

 

How about 225/50-18 ... Thinking about improving the ride quality with a taller side wall. Think these will fit?

Tire Size Comparison
Specification - Sidewall - Radius - Diameter - Circumference - Revs/Mile - Difference
235/40-18 - 3.7in - 12.7in - 25.4in - 79.8in - 794 - 0.0%
225/50-18 - 4.4in - 13.4in - 26.9in - 84.4in - 751 - 5.7%
235/50-18 - 4.6in - 13.6in - 27.3in - 85.6in - 740 - 7.3%