I am ordering new tires, but just can't choose what size to go with. Original is for Superb 205/55/16. For now I have 18" Flat Fives and 225/40/18 tires on them. As Estonia is quite known for it's crappy roads (waves in road), it's quite hard to drive so low tires, because car just wants to go along the wave.

So what have others got?

I got this graph to show, what could be used

http://ejelta.com/tiresize/index.html?ws18=1&tiresize=205%2F55-16&minwidth=&maxwidth=&minratio=18&maxratio=18&minwheel=16&maxwheel=18&maxdelta=2

Also do I have to check that new tires lie the speedometer + or -, if my speedometer is 50 and it really is 45 (measured by GPS).

I'm using standard 205/55/R16. Summer tires: http://www.reifen.com/de/TyreSize/DetailTyre/CarSummer/205-55-R16/V/False/False/a/205-55-R16-91V-EcoContact-5/15105851

205/55/16 are best for comfort and handling balance, if you need better handling it is best to upgrade shock absorbers rather than go for bigger wheels/lower tyres. Bigger wheels / lower profile tyres may look better for some people, but they increase harshness, noise in the cabin and wear of bearings, so in a car that you want to drive rather than show off it is always worth to go for smallest rims possible.

Note on faster Superbs (V6, more powerful petrols and diesels) the front brakes are 312mm discs and these require at least 16in rims, so 205/55/16 is the only manufacturer specified size that is both comfortable and possible.

If you feel like experimenting, you can also use 205/60/16 for even more comfort, it should still fit in spare wheel well at a push.

By the way, I had both 15in and 16in rims for my Octavia, and what was interesting was that pumped up 195/65/15 tyres were still more comfortable, less noise and better handling than the "fuel flap spec pressure" 205/55/16 tyres. I compared both using the same tyre type (Nokian H).

Actually the question was for 18" rims. I have original 16" rims with 205/55/16 winter tires, but I got 18" Ronal Flat Fives. I still would like some better look for the summer, but a little more comfort than from 225/40 tires. Actually handling is more superb with these 225/40/18's than 205/55/16's, but I would like to get some smoother ride from these rims.

So should I choose something with bigger sidewall? I know that there are others who drive bigger wheels like 17" in this forum, so I am interested that what do they chose.

If you want to keep the 18's then you need narrower tyres and with talller sidewall.

There is a good chance you will get away with 215/45/18 tyres as they're 9mm bigger radius than stock.

Or you can go 205/45/18 for stock size, though 215/45/18 offers more tyres to choose from and has taller sidewall which cannot be bad for comfort..