I have a 2007 Roush 500RC, its supercharged, and somewhere around 500fwhp. In march I'm going to spend another 3-4000 on it and I'm wondering about my gears? My tires are 305/40r22 thats 31.5" tall, I get 16-18 MPG on the highway.

How much of a performance gain would I get with 4.10 or 4.56 gears?

 

With 31.5" tires it's not worth the expense, and your highway gas mileage will suffer quite a bit. The only performance gain would be off the line, but I'm betting you are getting enough wheelspin as is.