4.10 gears

I have a stock 2006 Mustang GT and wanted to put in 4.10 gears. I was wondering how much it would hurt my gas mileage though. I'm not concerned about city driving at all but I am about highway. Just wondering what RPM the engine will be running when going 80mph.

If you have 2.73 rear end now, it will run 25% faster at any speed. Yes, this will affect the gas mileage out on the highway. Expect 25% less mileage.