I do have several comments about this subject. First, I think that except for our friend from down under, I am the only one with a second gas tank with a full skid plate. I had to relocate the spare for the extra 30 gallons of gas. I also run 265/70/17 tires so the old spare was a couple of inches smaller. I like the spare mounted on the back for several reasons. It does look cool, it gives me lots of comfort to know that I have 55 gallons of gas for long long trail rides, I have a portable gas station for my bud's on the trail, and I can easily check to be sure that my spare is not flat when I need it.
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In addition to carrying an air compressor, I would suggest that you get a plug kit. It has a tool that looks like a pointed scredriver with a hole in the end and you use it to insert a rubber and cord plug into the hole where you had a puncture along with sealant. It works and will get you off the trail without having to break the bead on a tire.
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