Is it completly necessary to have an 'over the axle' pipe when doing a cat-back exhaust? Can I not just run a 3-inch pipe from the cat to the muffler?

-TIA

 

Because the axle moves down as well as up during normal driving, it's hard to say how much room to leave beneath your axle and your exhaust. routing it above keeps things out of the way an also keeps stuff from getting tore-up.