I goofed, I have 4.56 gears and yes I use it only off road now, but with 38's I think they would be right for you.
If you are going to have them installed by a shop, you also may want to consider a rear locker and you will pay between $1,000. to $1,500. per axel at a shop.

 

I have a 2001 chevy silverado 1500 z71 with a 6 inch lift on 35s im lookin at getting it regeared and thinkin 4:56
But im kinda curious because i have heard alot all the guys at the dealers and the shops say it will run good and get good gas milage

everyone else says it will be very slow and get bad gas milage

can some one tell me how my speed and gas milage will be affected by this?

 

Check out this link to determine the rpm with the tire and gear change. http://www.ringpinion.com/Calculators.aspx

 

I would go with 4.88. In fact when I get my 35's I will be going with 4.88's

 

I wish I had gone with the 4.88 as well with the 241 transfercase in the Tahoe, my 87 K5 has the 208 transfer case with 4.10 and 35's and it seems to be equal in low range to the Tahoe.

 

I rode in a 1983 chevy 2500 with a 4in lift and 33" tires, buick 430 cubic inch motor and a built 400 trans. First he had 4.10 gears in it and it was quick, then he upgraded to 4.88 gears...wow what a diffrence. It sold me, my blazer is gonna get 4.88 geaing. Tons of torque and still pretty drivable on the highway.