My vehicle is a 2000 Chevy Silverado 1500, 4x4, Z-71 with the 5.3 engine and an automatic. It has 98,600 miles and I've added an Airaid throttle body spacer and intake tube, K&N air filter and a Flowmaster 70 series muffler with split duals.
Currently I get 12-14 mpg in town and 15-16 mpg on the road, if I'm lucky. The truck used to get 19 mpg on the road and 16 in town, and I don't know what the problem is. I've changed the plugs and wires, and had the fuel system cleaned with no improvement. I had a machine plugged in and it didn't show any codes, but shows an intermittent miss on 2 to 3 cylinders at low speeds. I think I have a miss at 55-60 mph or so.
No one I've talked to has any idea how to improve the mpg. This is my daily driver and any extra mpg would sure be great.
It's never easy to determinate the cause of a loss of a few mpg -- there are so many variables. But I do have a few things for you to check. Think back, did the change in fuel economy occur shortly after the addition of a specific performance add-on? Maybe the throttle body spacer which is said to increase performance by creating a "spinning action of the incoming air"?
The mystery misfire is a red flag. I'm not sure how the person who plugged in the machine (presumably a scanner) determined there was an intermittent misfire with no trouble codes. But if there is a true misfire, high speed, low speed or in between, have that diagnosed and repaired before you do anything else.
Have you replaced an oxygen sensor lately? With over 98k miles on the clock it may be time. Your 5.3-liter has four, one for each engine bank in front of the catalytic converters, and one for each bank after the cats. These sensors, especially the two up front, are what tell the Powertrain Control Module whether the exhaust gasses are rich or lean (low or high oxygen content), so it can control fuel delivery accordingly. Oxygen sensors have a significant affect on fuel economy, and they can get lazy as mileage accumulates.
Lastly, engine compression may not be what it once was. Hopefully that's not the case, but if were, a few mpg would not warrant major engine work.
I suggest spending an hour of the shop labor-time rate at a Chevy Dealer, and have them give your truck the onceover. Along with the fuel economy issue, mention the misfire you're experiencing and request a performance check of the O2 sensors.