I would replace the upstream O2 sensors. these old ones cause lower MPG with NO code failures. sensors get slow on reporting fuel mixtures then to run to rich. 100,000 mi is the recommended use of these.Sorry to revive an OLD post like this but I can't believe the things people come up with as far as "What do I do"? In this case, I think I would check the "E-F" gauge and make sure it's leaning a little to the right of "E". I'll be so damn glad when the new title comes in for my recent out of state purchase so I can unload it rapidly. Already have two offers from complete strangers to buy this gas hog, even after telling them what my actual fuel mileage is and they approached me just because of how clean this truck is. No 4-sale signs on it or anything. (15 mpg). 2000 Chevy 2500, 126,000 miles, old body style,(88-98?) 5.7, 4 speed AT, 3.73 rear end, 2 WD, absolutely NO rust, and this truck has NEVER been in salty winter time conditions as far as I can tell. Came out of northern Texas last July to Pa. I don't believe the previous owner drove it at all last winter in Pa. I wet my finger and rubbed it UNDER the lower door flaps and tasted "NO" salt. Kinda hard to get ALL salt residue out of that area. I've had this thing for 6 weeks now and have put 1,500 miles on it. Kept track of fuel use since day one. Best I got was 15.3! Damn near $400 in fuel makes me think that I could be riding in something a lot more comfortable and hire someone whenever I need something hauled. Sorry for the rant and I really love older trucks but this rough riding SOB has really turned me off! Hate to go foreign or hybrid but think of all the bucks I'd save....Naw, can't do that, I just need a smaller truck that gets better mielage. NSG
2500 uses more fuel / more weight than a 1500 ... my 1500 gets 18-19 MPG higher in warm weather / lower cold .
2500 should get 16-17 MPG highway at legal speeds .
If you do not require a truck I would consider a vehicle that gets high MPG for just traveling to work etc. buy a smaller rated truck if you need a vehicle to move things and just park it until you need it. fuel savings will be great if you travel high miles when using the car that can get 35 MPG .