Also, if you are sharing a spare with more than one trailer, make sure the bolt patterns on the wheels are the same. There is more than one 15-inch 5 lug bolt pattern out there, even with identical tires mounted on the wheels, as I discovered on a recent Sunday afternoon while hauling a 30-foot travel trailer across South Dakota. We were 400 miles in to an 800 mile drive heading back home when it happened. I was using two of my car hauler tires and rims as spares (I figured I would really be prepared with 2), I had blown out a tire last year and figured on replacing all four tires before next summer. I had been checking tire pressure each time I stopped and had just checked it 30 miles earlier, it blew and destroyed the side of the trailer before I could get stopped (no I don't think it was a stone bruise since it was the rear tire - though I had been going 75 mph all day). I whipped out my aluminum racing jack and star type lug wrench from the trailer compartment and figured no problem - until I couldn't get the rim of the spare to mount on the studs - the bolt pattern on the rim was a little too big. Unfortunately, in rural SD on a Sunday afternoon, nobody, including the RV dealer with the big US flag out in front of his building right by where I stopped, was open to mount the tire from my spare rim on to the trailer rim. Plus I had just discovered that morning that the trailer battery was bad and I had to keep the Suburban hooked to the trailer to keep the fridge running (with instructions to my wife to start it every 15 minutes so the trailer battery didn't draw down the truck battery. Luckily my daughter was following me so I took her car, with the tires, and left my wife, both kids and both dogs while I drove back in to Sioux Falls where I was able to find a Wal-Mart and buy a new Goodyear tire (don't know if it was a made in the USA one or not) and get it mounted. Luckily we didn't have any flats in the next 400 miles since we still didn't have a spare and it was too late for anything to be open then at all. I did notice that the Goodyear tire was running cooler than the Maxxis tires that were on there (lower pressure increase). Once I got home I figured out that my car hauler has a 5 x 5 bolt pattern and both the travel trailer and my wife's small cargo trailer have 5 x 4.5 bolt patterns. This has been an accident that has been waiting to happen for a long time since I have been using the travel trailer spare for a spare on my car hauler ever since I bought it, 6 years or so ago, and have pulled it all over the country!
One question I had was what about running 225/75 15 C range tires on the rims in place of the 205/75 15 C range tires that came with the trailer since they have a higher weight rating.