Just getting myself another set of 18" wheels to replace my current Mille Miglias. Now at the moment I'm back on my old 15" alloys which are wearing 195/60x15 Continentals.
After searching for a cheap source for some 215/35VR18 Pirelli Pzero Nero tyres I saw some Federal 215/40VR18 SS595 tyres which look to be not only cheap but have a fairly good write up, great grip etc but no mention of performance in the wet. As tyres are very important to me I haven't decided to purchase them yet, but at £146 delivered for 2 I've got to say I'm tempted. Anyway the tyre size didn't seem quite right so I decided to use my trusty tyre size calculator website and see what the result of fitting 215/40VR18 tyres would be on the old speedo.
Here's where it gets interesting, I input my existing tyre size, 195/60x15 radius is 307.5mm, radius of 215/40x18 is 314.6 giving a 2.309% difference and resulting in the speedo reading 71.62MPH at 70MPH. Ok I thought I can live with that. No figures were given for my existing tyres however 235/35x18 are radius 310.85 with a 70.76mph and 245/35x18 are 314.35 and 71.56mph.
If these figures are correct why do we stick with 215/35x18 tyres and not try other sections etc or am I missing something fundemental.
I can source cheaper tyres in some of the other sizes, what I'm asking is should I stick with what known to work or experiment a bit.