Should I be getting better gas mileage? Improvements?

[Supercharged111]

It’s not the only reason but it is a contributing factor. He went up basically 4 tire sizes. Either regear or go to stock size or very close to stock. He may get away with 265/75s with those gears and not suffer too much. And that big a tire will tank mpg. I went from 265/75/16 to 285/75/16 on my diesel 7.3 Ford and I’m priced a difference right away. My mpg dropped also. I went back to the stock size

The trans should be a 700r4 unless someone did a complete swap along with computer and harness for a electronically controlled trans. If it’s a 700 check the TV cable for misadjustment
Your tires are way too big for that gear ratio. Your trans isn’t shifting right because of the tire size.
The thermostat is keeping the engine on the cool side ( not in a good way) Sonora running open loop and dumping fuel
Trying to bring the engine to operating temp and it can’t so it’s running open loop.

Install correct temp thermostat I believe 195 *
Install normal size tires. Yeah it won’t look as cool
Recalibrate the computer for a 350 ci and get the right injectors.
30 psi is too low. Your tires are under inflated.

4 sizes? That's one size bigger than stock for my 1500. And even a 160 stat will have you in closed loop. Is this 100% city driving? Try 3rd gear on the highway and see what you get.

 

[1989GMCSIERRA]

4 sizes? That's one size bigger than stock for my 1500. And even a 160 stat will have you in closed loop. Is this 100% city driving? Try 3rd gear on the highway and see what you get.

Well 4 total metric sizes. Either way for a truck that would come with 245s that are 31s he moved to 33s and he’s running basically highway gears. He needs to be at 3:73/4;10s
Your truck may have different gears so that’s why it came stock with a optional larger size. Just because your truck came with 265s doesn’t mean it’s ok on his truck. You may have 3;73 and 265s.

 

[1989GMCSIERRA]

In post #1 the OP stated it's a 93 with a 4L60E.

I had a 93 service truck and it had a 700R4. I didn’t think the 60E come Out until 1994/95 on trucks.

 

[98chevy2500SS]

Skip the 4.10 gears and go for gears that will keep you @ 2300 to 2500 rpm at your cruising speed on the highway. The 4.10:1 gears would be O.K. if you didn't have overdrive.

The 3.42 in OD becomes 2.39 and 4.1 becomes a 2.87. Even 5.14:1 becomes 3.59. I often recommend 5.38:1 for tires like you have.

The truck is under-geared for the stock tires and the taller ones compounds the problem.

4.10s with 285/75r16 (same as 285/70r17) w/ O.D. do not run 2300 to 2500rpm. At 55mph, mine runs 1800rpm. Can't quite remember what 3rd gear runs at since I rarely use it, but I know it's higher than 2300, which would suck if you didn't have O.D., because you're burning more fuel than if you didn't have it. In that case, overdrive would benefit.

"In Town" mpg.

Thank you. City driving being the key words here. 9-10 for city is about normal, highway you'll see better mpg, so I'm not surprised OP is seeing about 9.5 for city.

 

[Selrahc360]

Install a proper thermostat. Lose the bigass tires and any excess weight.

Connect a scan tool, find out what the computer is trying to tell you.
Fuel trims, knock sensor, spark advance--verify all the sensors and computer outputs. There's a reason the idle is way too high, and that's why it jerks when you put it in gear.

No functional EGR probably means the knock sensor is going nuts, and you have mega-retarded timing. Fix the EGR.

Make sure the brakes don't drag

To old for Obd2

 

[Selrahc360]

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!

Besides the tire's, it is your engine set-up. Which means some work. You're cam needs to be swapped for lower rpm power. Why I know this is currently putting in a 383 stroker in with a tow came for camper duty. Why the stroker bigger piston and longer stroke equals torque. Why GM did not even do this? Probably because of the 454 the tooling was not used up yet and would have cut the margins. Anyway if your torque band is lower you will spend less time squeezing gas into the engine to get going, you will give up top end, far swap for better fuel mileage with gas$$$

 

[L31MaxExpress]

Besides the tire's, it is your engine set-up. Which means some work. You're cam needs to be swapped for lower rpm power. Why I know this is currently putting in a 383 stroker in with a tow came for camper duty. Why the stroker bigger piston and longer stroke equals torque. Why GM did not even do this? Probably because of the 454 the tooling was not used up yet and would have cut the margins. Anyway if your torque band is lower you will spend less time squeezing gas into the engine to get going, you will give up top end, far swap for better fuel mileage with gas$$$

GM DID make a big inch small block, the 400.

 

[PlayingWithTBI]

To old for Obd2

??? there are plenty of scanners and APPs for laptops that work well with OBDI ECM/PCMs

Here's an example of TunerPro RT on a laptop logging a 1227747 ECM in an 88 C1500

You must be registered for see images attach

With monitors too

You must be registered for see images attach

 

[PlayingWithTBI]

I didn’t think the 60E come Out until 1994/95 on trucks.

GM came out with electronic controlled transmissions late 92 to 93. The 454SS had a 4L80E in 91 IIRC.

 

[Nad_Yvalhosert]

To old for Obd2

But good scan tools can still give you access to the datastream