Hell - Sometimes they steal the rest of the truck too.
They like to steal spares because they don't have to block the truck up, and most people don't notice its missing right away, giving them time to abscond.
If you want to slow them down, add a second lock/chain, one of those door type alarms that would go off when the tire is let down, etc...the more time it takes them, and the more likely they might be caught, the less fun.
For tire carriers, I'm always torn between wanting to be able to quickly get the spare on/off on the trail, and wanting it to still be there in the city...My company has had winches, lights, tires, you name it, stolen while parked in the city...and it seems to be all about advertising....
If the passing purloiners notice something of value, they home in on it...if the winch is behind the bumper, or the lights are folded down, etc...they don't see it like they would if it were in their face as they drive by.
Spares on the other hand are low hanging fruit....they know that they're under there, ripe for the picking.
Parking with the vulnerable parts facing the light/public view (If they take it, people will see them....) works in areas with morales (Stop, thief!), and back fires in areas without, as you are merely displaying your wares for the taking.
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- TJ
2001 Xterra '03 VG33, SE 5 spd, 305/70/16's, Revolvers, UBSkidderz, Doubled AAL's, 3"SL/2"BL, winch/bumpers, skids, sliders, OBA, Snorkel, pine stripes....
Friends don't let friends drive stock.
http://www.gifsoup.com/view/501230/tj-tackling-crawlers-ridge-o.gif