Tyre Sizing BA2 XR6T Ute

[vegie]

Hi all,

 

I have a BA2 XR6T ute.  The tyre placard has two tyres listed - 245/40/18 (which I have on the car), and also 235/45/17 - both on 8.0J wide rims.

 

When I go to tiresize.com to check out the difference, I see that the diameter of the 17 inch is 10mm smaller than the 18 inch.  Whilst minor, this would mean speedo error, and I guess I might get slightly better acceleration with the 17 inch in this size (although this is not the reason to compare).

 

Now, when I play with tiresize, I see that a 245/45/17 is in effect near on identical in size to the 245/40/18 (except for the greater sidewall of course).

 

So, why would Ford have the 235/45 listed as the tyre size for 17 inch which means the tyres are different diameters, when the 245/45 would mean an identical tyre size for both 17 & 18 inch?

 

Would there be any issues fitting a 245/45/17, firstly from a 'fitment' perspective, and secondly from a legal perspective?

 

TIA.

[arronm]

Ba bf Xrs came 17s. 18s was an update.  To get you Speedo correct you need to install the tyre size it came from the factory with.  Or use 245.40.18 and check with GPS.  Speedos are always fast from the factory.  So the larger tyre od will just make it read more correct