Probably been asked before.

I'm wanting to change my rear diff gears to a 3.73 or 4.10. Haven't decided.

I have 3.08s right now...

How do I tell what size gears I need to get? 8.8"? 9.75"?

Also, What is a good brand of gears and where are places I can order them?

How big of a mpg difference will I see between the 3.08s and the 3.73s? 4.10s? Keeping stock sized tires probably forever. No plans to change wheels or tire.

Is there is a big difference between 3.73s and 4.10s mpg wise?