How much would this mess with speedo ect...
Makes sense to me that it wouldn't 40 series tires are proper sized for 18" wheels. Though you might run into rubbing issues with the 235 when lowered.
You're speedo will you going 2.1% slower than you really are, depending on the rims your running and how much you're lowered if you are, you probably won't rub...
Tread width also effects sidewall height, a 40 sidewall i believe is 40% of tire width.. so the wider you go the higher you go also.
i think you should be ok.
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- Sold my beloved J in April 2010 -
I'd stick with 225. It'll save you any potential issues.
A 235/35/18 would actually be closer to stock size than the 235/40/18. With the extra width, I think you'd be better off erring on the shorter rather than taller side since it will give you a hair of extra clearance on the fenders if that becomes an issue.
