Well, keep in mind that the 2nd number is a ratio... so a 235/40 is going to be a lower profile tire than a 245/40... Also, a 245 on a 7.5" wide rim is pushing it... in fact I think on my roommate's 235/XX-17 ST-115 have a warning printed right on 'em "do not install on a 7.5" rim".
I'd check tirerack and see how man rev-per-mile each size tire has and what's closest to stock... if they're both pretty close, go with the wider tire as long as you know there's no rubbing.
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