Tyre width affect overall diameter?

[Mk2]

I'm just looking at putting some 18s on my VX and all the calculators online state I will need 245/40R18 in order to best match my stock overall diameter of 225/55R16. I was just going to buy some 235/40R18 because they were cheaper but it will apparently put my speedo out by 1.x% rather than 0.x%. I didn't realise width of a tyre impacted; I always thought it was just the height of the sidewall in comparison to the wheel. Can you guys shed a bit of light on this for me?

edit: Wait, just to clarify something, I'm an idiot. The 40 is just a percentage I have now found out, so that would automagically mean the 245 does have a larger sidewall than a 235, yes?

Also, I am putting the tyres on WH Grange wheels which are 8" wide. 245 too big?

[thewheeldeal com au]

Yep, you've got it right - profile (or aspect) is a percentage of width which means as width increases so does the sidewall height.

245/40R18 will be fine on your 8" wheels.

[Mk2]

Thanks mate.