so i bought some used 315-70-17 BFG All terrains and one of which was an unused spare, my question is can i buy three new tires and get away with using the spare or has its unuse comprimised the tire? the tires were a few years old. is it safe/smart or should i get 4 new tires, TIA

 

That's a toss up. It depends on how and where it was stored as a spare. If it was stored under the truck I would think you can as long as no dry rotting is evident. Should be fine.

 

When I rotate my tires I always do a 5 tire rotation.. However as stated above I think it would depend on how long its been up there.

 

Also check the load rating on the spare tire as it could be different than the other three. If the tire is in good shape, meaning tread, dry rote , load rating you should be fine using it.

 

Many people get the wrong info on tires and think that just because tread is good the tire is OK. Any tire 7 years or older even if they have never hit the ground should be disposed of as the rubber can separate and they can come apart at moderate speeds.
Story was on 60 Minutes on CBS.

 

Living in Phoenix where it gets to 120 degrees in the summer, I replace my tires every 3 years regardless of remaining tread. The heat is very hard on them and I've seen substantial dryrot in 3 years. I ran a set on my F-150 years back and during the fourth year I had a catastrophic blowout. My safety is worth the extra caution. Most of my previous trucks I've chewed up the tires pretty good after 2-3 years. The stock rugged terrains will have about 30K on them when they hit the 3 year point, plenty of tread remaining. Since the truck spent it's first 2 years in Colorado I might run this set to 4 years.

 

What Bronco and Bearwhiz said. All I ever do with a spare is make sure there's air in it. After a few years you can't trust them, no matter what. All I would ever expect out of it is to get me to the next tire place down the road - going SLOW. I have a spare wheel with no tire on it, that will get a new tire mounted just before my next long-distance hauling trip (more than a few hundred miles), whenever that happens.

 

best is to check the out side for weather checking and craking, but more importantly is the inside carcase.