Originally Posted by
Nexus
You need to check with the manufacturer specs for the particular tyre you are considering.
There are very few 225/40R18 that are warranted by the manufacturer for fitment to 18x7 rims. Likewise there are very few 235/40R18 that are warrantable for fitment to 18x7.5 rims
Each tyre manufacturer publishes specifications for the tyre, and this is the final word in what is or is not roadworthy if it passes all other transport authority requirements.
A good example tyre for which 225/40R18 will fit to 18x7 rims and 235/40R18 to 18x7.5 rims is Federals SS595
If you check Federals website, you will see of three different SS595 variants the entry level SS595 is good for these fitments. If you then check the street performance and track performance variations on this tyre (SS595-EVO, SS595-RS) you will find that these same fitments are not warrantable and therefore illegal/unroadworthy.
Generic fitment guides have the potential to mislead - don't use them. There are many generic fitment guides that say 225/40 is OK on the 18x7 OEM rims. The problem is that 90% of manufactured tyre specs disagree and specify a 7.5" minimum rim for the same tyre, so the generic guides are giving the OK to an unroadworthy tyre fitment. The manufacturer fitment specs are authoritive. The generic fitment guides are not.
Always check with the tyre manufacturers published specs and you won't go wrong.
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