Syncromesh gears have a helical or angled cut - the teeth arent parallel to the axis the gear spins on. They'd look like this:
this makes the gearbox quieter, as the teeth don't "slap" together as loudly as the gears drive one another
Strait-cut gears are cheaper, simpler, and ARGUABLY stronger - they look like your standard conception of a cog or gear wheel. They're pretty much de rigeur in race cars that don't have stock-gearbox regs. As most race cars have easily replacable gearing - to accommodate different track needs, engine tuning, etc. with many ratios to maintain, a big box of the straight-cut gears are a lot cheaper. they're also lighter, as the teeth of a helical gear have to be longer to reach all the way across the gear wheel itself - more metal, more weight.
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[This message has been edited by cvalle-sd (edited December 19, 2000).]