Tire size for a lowered car
 

A year ago i bought a 2.7 manual 2002 boxster, it has been lowered, dont know the details and has 18in wheels. The car is only about 6cm above ground. The ride i find a little uncomfortable and have to be alert of portholes. Am thinking of putting 17 in rims back on the car, and i checked with miata website the size of 17 in wheels are bigger than an 18in, by one inch. My question is will the ride be more comfortable with the new wheels? Currently F220/40/18 and R265/35/10 to be replaced with 205/55/17 and 235/50/17.

wheel-size.com has a great comparison tool to get exactly what you want and yes a 17 will ride smoother on rough roads than an 18 will.

The correct tyre size for 17" wheels are 205/50 ZR17 (F) and 255/40 ZR17 (R).

Using the sizes you suggest will increase the overall rolling circumference and increase the radii of the wheels. That will slow your acceleration & turn in, though they will give you a softer ride.

First, the car is too low and will handle like poo. Suspension geometry is all wrong with the car so low.

With the car at a better ride height, 225/45/17 and 255/40/17 will work well. Mount Bridgestone S0-4 tires for a nice comfy ride that still has good grip.

Yes i agree with you about the ride, its terrible although has great accelaration.