I do a lot of highway driving. But, right now I've got a 3.31 open rear end. And I will be installing my ATI soon. I just want to know if the 3.73s will hurt my gas mileage on the highway? Will I like the difference the 3.73s make? Right now I'm running 245/75s.

 

No, 3.73s are not too low for highway speeds. Yes, they will hurt your mileage during extended highway use. Absolutely-Positively, you will like the difference coming from 3.31s !!

 

Going from 3.31 to a 3.73 will bring your rpm up about 12.7%

3.73/3.31=1.126888217522659

Ballpark figure is if you are running 2,000 rpm at 70 now, the new gear will bring it up to around 2250 rpm.