currently have 20 x 12 on rear, 20 x 9 on front

It needs new tires, because the rears are 20 x 12
so 305/25/20 or 315/25/30? will a 30 series fit ?

alternatively, please give me a nice setup for 19" wheels/tires

now would be the time to change if needed. Ive read that 25 series tires are too bumpy..... thanks

 

I had 18" AMG stock and went to 19" a few years back, the ride was a little firmer, but also better on corners. I would think you moving to 19" the extra rubber will make a difference to the smoothness!
I went with 265 x 35 rears and 255 x 35 fronts. On the rears I used 15mm spacers and the fronts had 10mm to move them out to the wheel arches. I also lowered my car by 35mm on Eibachs with no rubbing,


18"

19"

 

I'm no expert but, I'd go with 255/30/20 at the front and 305/25/20 at the rear for better accuracy.

 

The main limiting factor for R230 tire sizes is the clearance for front tire diameter. Sounds odd, but here's why.....

You don't want your front and rear tires to have significantly different diameters because it would cause problems with the anti-lock braking and traction control systems. So you have to consider front and rear tire sizes as a matched set.

The maximum diameter/height of the front tires are limited by the upper control arm ball joint bolt. The head of this bolt protrudes from the underside of the control arm and so limits the diameter of the front tire. I've heard of people grinding this bolt head down a bit to gain extra clearance, but I personally wouldn't do that, or recommend it.

So basically, once you calculate the diameter/height of your front tires, the required size for the rears falls into place. Since the rears are wider, their aspect ratio must be lower in order to maintain the same diameter as the fronts. As @Abebundance suggested, a 255/30/20 for the front will have a sidewall height of 76.5mm (30% of 255 = 76.5) and 305/25/20 will have a sidewall height of 76.25mm (25% of 305 = 76.25). From what I've read/heard, you don't want to have any more than a 3% difference in front/rear tire diameter, so you have to account for sidewall height x2 plus the diameter of the wheel. Or just refer to this tire height calculator.

Lastly, most tires are not available in every possible width/aspect ratio/wheel diameter, so you may have to settle for a few millimeters less than the theoretical maximum that would fit the car. If you're going for *perfect* tire-to-fender lip spacing, you may have to settle for a slightly smaller than perfect tire width and use spacers.