4.10 gears

But to answer your question it is up to you to determine how much it will effect your gas mileage.
Just shift normally and compensate for crusing speed with 5th or 6th gear and you wont see a difference.
You'll still get decent gas mileage. It's a manual. You can drive it however you want. Try a search, you should find quite a bit of info on this subject.

