I've got a 2000 Sierra Classic crewcab swb 4wd with a 5.7l/4l80e and 3.73's with factory size tires. I can't get any better than 12-12.5mpg with 90% highway/turnpike driving. Is this pretty typical with this combination? I have spent roughly $600 on add-ons (new 2.5" dual exhaust with balance tube, Jet power programer, and a K&N cold air intake) to try and improve mileage with no results. Any suggestions?

Do you have a locking gas cap?? I have basically the same truck with 112000 miles and I get over 20 on the highway and around 18 in town and got around 10.5 towing my sons race truck in a 24' enclosed trailer. Everything is factory stock. Maybe check your speedometer/odometer to make they are reading correct.

Mikey could you explain to me how the locking gas cap would affect mileage? TIA

well if someone was stealing your gas...

Do you have a locking gas cap?? I have basically the same truck with 112000 miles and I get over 20 on the highway and around 18 in town and got around 10.5 towing my sons race truck in a 24' enclosed trailer. Everything is factory stock. Maybe check your speedometer/odometer to make they are reading correct.

No locking cap. I highly doubt that is the case considering where I park it and the fact that it is dead on consistent.

When you say "bassically" the same truck. What are your specs? I have seen a huge difference in mileage over a minor difference depending on what it is.

My speedometer is dead accurate with my GPS.

Anyone else running a 5.7l/4wd/3.73 crew cab 3/4ton?

You are right my truck isn't like yours. I didn't see the 3/4 ton in your original post. sorry