When we originally designed the X - 5, back in like 2003/2004, we wanted to use the XXX-4 spur gears, either the 92 or the 94 spur. Unfortunately, it was just impossible to get the gearing right with that small of a spur - because the internal ratio of the car is only 1.75, with a 94 tooth spur gear we ended up using like 13 or 14 tooth pinions. The larger spur gear really opens up the gearing options.
So, since we had to make spur gears, we went and found the best gear-machining company in RC: Precision Racing. They do charge us a little more, and thus the spur gears are a little pricey, but if you treat them right they'll run forever. I've used the same 3 spur gears (a 102, 104, and 106) for the last two years at least.
I suggest really taking time when you set the gear mesh. Be really careful as you tighten down the motor strap, as it can sometimes shift the motor one way or the other as you tighten. I usually tighten the bolts together - one turn on the front one, one turn on the back, one turn on the front, etc - and examine the gear mesh frequently as I tighten. Make sure your pinion gear is in good shape too - I don't usually think of pinions as wearing out, but just this last summer I took a hard look at my pinions and replaced a lot of them (they were like 6 years old, lol).
So take some extra time setting the mesh, checking often as you tighten down the screws, and the spur's should last a good long time.
If you're the adventurous sort, you might check into finding some on-road spur gears from a touring car. You might be able to find one that you could make work; I don't know. Not sure I'd recommend it as the gears are 64 pitch and thus even more fragile, but the gears themselves might be cheaper.
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