i drive an 03 chevy silverado and im not sure what the factory gear ratio is for the rear end?! and also if i were to get 4.10 gears, how bad will my gas mileage drop???

 

One is left to assume quite a bit of information here. Assuming you have an 03 Silverado 1500 without any special packages, 4WD, and either a 5.3L or 4.8L Vortec with a 4L60 trans, you probably have 373 gears. Could be 342s or 410s stock depending on your engine and truck size, and various packages.

I know when I shifted from 410s to 342s for all my highway driving, I wound up saving a significant amount of money by dropping 1000 to 1200 RPMs at highway speeds. About 30% overall. It costs about 1500-2000$, so a CAI might be a more efficient alternative.