Fuel Mileage?

What kind of driving? What rear end? Lots of error in one tank. Could be about normal. Straight highway driving you might expect 18 or so at the best.

My 95 1500 2WD got 18- 20depending on driving conditions. Had the medium range rearend 3:42 or around there. Definately not the 4:10's those are lucky to get 10 Mpg.

Not everyone's idea of a "complete tuneup" includes an O2 sensor. Replace it. Don't throw the original away, especially if you purchase an offbrand cross-reference replacement. Bosch and AC/Delco are good. In any case holding on to to the old one a month or two is cheap insurance.

As others have said 10 MPG might not be too far off the mark. There have been some real MPG improvements in recent years. Things that you can not retrofit.

how do I determine which rear end is on the truck?

Replace the 02 sensor.

Doc

Look in the glove box for the production sticker; it will have lots of three letter/digit figures. Look for one that starts with GU or GT followed by a number and post it here.

Doc

Here are the specs:

GU4 Rear axle, 3.08 ratio

GU6 Rear axle, 3.42 ratio

GT4 Rear axle, 3.73 ratio

GT5 Rear axle, 4.10 ratio

G80 Differential, locking, heavy duty

G86 Differential, limited slip, heavy-duty, rear

GM has introduced a new rear axle configuration on its 2005 trucks:

GU5 Rear axle, 3.23 ratio