uhm.. i'm not so sure about that. i thankfully don't have any practical experience with these things, but i was under the impression that sugar in the gas tank would do a lot worse than make the car stop. see, sugar burns more intensely than gas. makes nice big hot shooting flames. again, i'm not sure, but i was told that sugar in the gas tank could produce an oversized explosion in the cyllandars, seriously damaging the engine and maybe even igniting the line back to the fuel tank.
i think your idea of a damaged belt is a better one. wear it thin in one place, nick it a bit. the belt goes round and round as the car drives off, and that weak spot wears thinner and then snaps, a few miles down the road. if you do it right, it'll look like ordinary wear and tear or maybe even a previously unnoticed defect.
only other suggestion i can give you is to mess with the electrical system. if you damage the alternator (the bit that connects the engine and the battery, so that the battery can recharge), the car will work for a while on battery power. you'll get a little light on the dashboard, like "check battery." it's easy to ignore that little light. the car seems to be running fine. but then the battery slowly drains, and, a few miles down the road, the engine gives out, the lights and clock and radio and things inside the car die down, and the car coasts to a stop. (this, btw, is exactly what happened to mom and i this one time when we were driving along the highway and the alternator burned out)
not sure exactly how you'd tamper with the alternator, but it shouldn't be too hard if you know what you're doing. i think if you just thin the wire, it'll overheat and melt down, and, in doing so, remove evidence that it was tampered with. it'll just look like a defective wire. or you could do something that would cause a short, and try to make that look natural (and, again, assume that when the thing melts down from the short, it'll be all but impossible to tell exactly how and why the short happened).
Paul
p.s. just did a web search. it's hard to find reliable info, but the best i can find is that sugar in the gas tank will carmelize in the engine, gumming it up. however, the same site says that it can also carmelize in the fuel line, clogging it. in either case, it should stall out, but i'd still be worried that it'd be unpredictable. not to mention that you don't know how quickly the toffee will disolve. oh, and it'll be fairly obvious what whappened as soon as someone checks out the engine. but maybe that's what you want. sorry... i'm half asleep at the moment, so i hope i'm making enough sense to be at least somewhat helpful.
