Based on my experience with my 02 (I've had exposure up the ladder for front end issues) I've learned the following:
1. The further up the ladder you go in a warranty question, the more "by the book" they get. DC officially warrants 265/70s. period. Don't expect to get any sympathy from on high for front end parts if you are running 315s.
2. Tire sizes are definately one of the first things the zone rep and others will ask about if you come in with a front end issue like tie rod ends, steering box, whatever.
3. The dealer service manager has some authority to represent you and perform a repair under warranty without involving anyone above him. Get on his good side and ask what he is confortable with. Mine is comfortable with 285s and has represented me wonderfully well through my trac bar issues on the 02. The value of a good relationship with your service manager cannot be over stated. I've heard of one other dealer service manger who was confortable with 315s on the 3rd gens, but I bet that is rare and short lived if any issues crop up.
4. A lot of guys run 315s on the 3rd gen trucks. the front end is a lot more robust than on the 2nd gens, so the risk is low. That said, consider playing it safe with a tire approved by your service manager. wear those out and then put something else on. By that time the suspension warranty will be expired anyway and you will have ironed out any issues previously.
5. running 315s without a re-cal will raise spedometer fraud questions if somebody is passionate about finding fault with you. The error is on the order of 10% (depending on the tire) which is substantial. If DC finds out you have 35,000 miles on a set of 315s and you come in for front end problems, guess what -- you really have more like 38,000 on the truck and so you're out of warranty anyway!
6. running 285s without a re-cal represents only a 3% error and does not raise the above issues.
Just my opinion but I think 315s are too tall for 3. 73 gears anyway. Engine rpms at highway speeds are too slow. good choice if you have 4. 10s though...
I'm not surprised that if DC finds out someone else has re-cal'd the spedo that they slap a limited warranty on the vehicle. YOu don't want anyone playing with the ECM unless you are prepared to be your own warranty station. Another alternative is after market spedo calibrators that live on your vehicle and re-map readings correctly. No change to the factory ECM, no loss of vehicle warranty, but if the after market device screws up something, be prepared for an interesting show. I don't think that has ever happend, by the way...
That happy situation aside, if you are committed to 315s, I'd have the dealer re-cal for you. Find one who will do it. Then you avoid all spedo fraud issues; even warranty claims issues. I'm told that the ECM will accomodate 315/70s dead on (will go down to 599 revs per mile. the BFG 315/70 is 601), Then, you don't have a limited waranty, and any parts failure disputes will be governed by the MM warranty verbiage, meaning that they have to prove that your large tires caused the failure. just an aside here, but fortune is not on your side in that situation -- large tires place additional stress on lots of things including the axle, so DC probably will have a field day showing that the larger tires were to blame. So you'd be in for an interesting fight, and probably on your own but really the risk is very very low -- guys run 315s and larger tires all the time and don't break things, even on 2nd gen trucks with 3. 54 gears. you just have to be prepared for DC to deny warranty coverage if your axle or a U joint breaks. Which is why I suggested running an "approved" tire first to avoid question. you don't need the tire factor against you when fighting a quality issue with the AA axle!
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