Okay, so I've been battling bad gas mileage ever since I got my truck a little over a year ago from my dad. He also got it from his, so I know everything ever done to it for the most part. It's a 1993 SCSB GMC Sierra 1500 with a 5.7l and the 4l60e. Its got ~241k miles on the body, 130k on the 5.7l which was swapped in for better towing power than it's original 5.0l. It also has a newer 4l60e, with approximately 30k miles on it. Side note: it will shift hard from 1-2 unless floored. I've checked my TPS sensor voltage and it seems fine. Before the original transmission was replaced, it had a shift kit, so maybe during the replacement the PCM or line pressure stuff wasn't adjusted back? Any help on that would be appreciated, but it never honestly affects my driving and it's not banging into gear. Also lurches when put in park and reverse. So, for the mileage... I get about 9.5 mpg city adjusted 10.5% for the difference in revolution/mile with tires. I have 285/70/17 (~33in) Yokohama Geolandar MT03s on GFX-TR19 rims with 3.42 gears. Aired up to ~30 psi all around. No lift, just cranked to level. My exhaust setup is the stock one for the 5.7l, with no muffler into a non-mandrel Y-pipe side exit, with a stock (I'm guessing replaced w/ the engine) cat. In search of better MPGs, I've switched from a 165 degree thermostat to a 180. There are no coolant leaks whatsoever. I've also replaced the engine coolant temperature sensor, O2 sensor, all engine and driveline fluids with synthetic, spark plugs (which honestly I forgot to gap) and wires, distributor cap and rotor, PCV valve, ignition coil, air filter, alternator, fan clutch, fuel filter, brake light switch, oil filters. I've done the IAC valve after getting a code for it, which still turns on when idling, and then turns off when driving. I did the relearn procedure, and it still persists, however doesn't affect performance at all. Idles ~1100 in park and neutral, ~700 in gear. Anybody know why? My EGR vacuum line from the solenoid to valve also broke and I've yet to replace it, but would it make a big difference at non-highway speeds? The TBI system isn't the cleanest, I should probably spray it down with throttle body cleaner. The injectors have a great cone pattern no drips. Back to mileage, my truck weighs ~5200lbs with a loaded toolbox, headache rack, custom off-road bumper I built plus a 12k winch. The truck runs like a top so I'm not sure why it's so low. I don't expect great mileage out of her, but 9.5 seems poor. I try not to drive too aggressively, but I occasionally have fun
. Any knowledge would be appreciated!