What width tire do you want to go with?
For the Accords it is usually recommended to go with 215/45/18. Personally I prefer 225, so then the size would be 225/40/18. Be sure to get the correct wheel width, though. I'm not sure if 7.5" or 8" is the correct for our Accords.
The correct offset will help minimize the risk of tire rub. If the tires stick out too much you may need to roll your fenders for them to fit. I think +42 and +45 are the usual recommended offsets for the 6th gens, however I'm not sure which one is preferable.
I'm still rolling on stock steelies and plan on eventually getting some 18's myself. Best advice I could give is for you to try to get in touch with someone rolling on 18's and ask them what width, offset, tire size, etc. they have. Also how much they have lowered matters as well. Then, when you're done with that homework assignment, be sure to report back to me with what you find.
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