Has anyone ever used these after market spark plug wires that are 9.3mm. is that too thick for a 97 civic and could it decrease gas mileage?

 

I don't see how it would decrease fuel mileage. Plug wire have nothing to do with how much fuel is injected into engine. Same amount will be injected but more spark would burn it more efficiently. I doubt they would help nor hurt unless you have something to support it. IE MSD ignition

 

if you put the sticker from who makes them on the car you get 5 hp