What do after,arket spark plug wires do for your car? I have a 96 escort wagon and they have 8mm and 8.5 mm wires. If they are worth it to buy what size should I get?

Buy the larger wires. I have 9mm wires on mine. I got them from Wires4Cars.com and they are top quality. I kept burning up the smaller wires.

IMHO If your engine is stock then I don't see why a bigger wire would make a difference. If it did then Ford would put them on at the factory. If you run fatter wires to a table lamp it won't get any brighter. That's my theory :p