[Mos6689]

Thanks for your advice Xmetal. This throws my understanding off a bit now. Bare with me here.

I may well be wrong but this is how I was calculating it:

So OEM setup is:

18x8 225/40 RD (rolling dia) 637.2mm fronts
18x9 255/40 RD 661.2mm rears

OEM difference fronts to rears + 24mm

Stock they’re both running 40 and the rears are wider. So there’s a difference front to rear in the rolling diameters from stock. So I was looking to get as close to the difference between the rolling diameters from stock as possible.

Back to my initial thoughts on sizes, I did take RD into consideration and here’s stats on them:

19x8.5 235/35 RD 647.1mm fronts
19x10 265/35 RD 668.1mm rears

Difference fronts to rears +21mm

What you’re suggesting:

19x8.5 235/35 RD 647.1mm fronts
19x10 265/30 RD 641.6mm rears

Difference fronts to rears - 5.5mm

OR

19x8.5 245/35 RD 654.1mm fronts
19x10 275/30 RD 647.6mm rears

Difference front to rears -6.5mm

Are you saying it’s better to have the RD front and rear as close to eachother as possible? If so, what benefit does this offer from OEM setup?

I understood changing the difference in RD from front to rear is what would throw up problems like speedo error at +\- 2.5% & possibly the ABS/traction faultering.

Be interested to hear what you and others think of this with the numbers. In parts of Europe (Germany for example) it’s illegal to have a speedo difference of +\- 2% when changing wheels. Also RD must be within a certain tolerance of OEM which only allows you a specific set of tyre sizes to choose from. Thought it was good practice to follow this but I’ve only recently got my head around all of this wheel jargon. Always just bought a set of rims and chucked them on without a second thought. Only matching PCD & nothing else. So I may well be wrong in my thought process.