Should I be getting better gas mileage? Improvements?

[Nad_Yvalhosert]

Is your speedometer corrected for the altered tire size? If not, the distance traveled vs what your speedometer considers a mile arent the same.
Oversize tire will make you travel more than a mile, for every "odometer mile".
That also means you're likely getting better MPG than you think.

 

[cheetahranger]

Is your speedometer corrected for the altered tire size? If not, the distance traveled vs what your speedometer considers a mile arent the same.
Oversize tire will make you travel more than a mile, for every "odometer mile".
That also means you're likely getting better MPG than you think.

Yes, it's 10.5% more revolutions per mile than the stock 225/75/16s. So when I measure from odometer I get about 8.5, so multiplied by 10.5% in reality I get about 9.5mpg. Don't mind less odometer mileage either if i decide to sell her in the future

 

[Frank Enstein]

I disagree wholeheartedly with the guy who recommended 5.38s for your 33" tires. Gm designed these engines to get better mileage by turning slower. As long as you shift into overdrive and your torque converter clutch locks up, your gears are fine. Maybe a tad tall, but not ridiculously so.. since you're only running a couple inches taller than stock.
The roof rack and bumper and winch all add both weight and wind resistance.
I think the high idle is telling. Does your truck smell like raw gasoline at idle? Is it hard to start when hot? I think your computer isn't seeing your coolant temp sensor. I had a bent connector on my most recent tbi truck. It plugged in but didn't quite touch the pins on the coolant temp sensor. The computer thought the coolant was -40⁰ and was absolutely dumping gas in on the coolant temp enrichment circuit. That would also increaae your idle speed like a choke in older engines. As the engine hears up, the idle should drop to about 625rpm. The gauge and computer use different sensors, so your gauge may read fine. Another factor may be your thermostat.. I recently changed back to a 195⁰ thermostat. I thought a 180⁰ would help performance but the computer doesn't really like it that much. It only keeps the engine from going into lean cruise mode because the computer thinks it's it's not fully up to temp.
The torque converter clutch won't lock if the coolant temp is too low.. so if the coolant temp sensor isn't reading correctly, the tcc won't lock and you'll be using more fuel.
If I'm going for maximum mileage, I air the tires up to max pressure or even a few psi over. I've never heard of dividing the weight by the max load and dividing by the max pressure before. That assumes that the load capacity increases linearly with pressure. And an empty truck isnt that heavy on the rear so you'll be underinflating the tires... Isn't that how Ford Explorers were blowing all of those tires in the 90s?
The stock tbi timing table is extremely conservative. I added between 4⁰ and 22⁰ spark advance to my 92 c1500's tables. Wide open throttle was set for 16.34⁰. That's incredibly inefficient. These trucks typically like 34⁰ of total timing. Unless you can get a custom chip or tune the timing tables yourself, I'd set your base timing to 6⁰ instead of 0⁰ and see how it goes. If you hear pinging, back it off. Proper spark timing has a huge benefit for both power and economy. I tuned mine over the course of a couple weeks, datalogging the drive back and forth to work, then making small tweaks. If I ever saw activity on the knock sensor, id immediately pull 4⁰ from that cell.
My truck has headers and free-flowing exhaust. That's the only engine modification. It's a 92 c1500 rclb. 350tbi. 700r4. 3.42. 235/75r15s. I tuned the tbi computer with my laptop. I'm getting pretty good mileage with mine once I've got everything dialed in. Gm should have tuned them like this from the factory.
As far as gearing, you can go with 4.10s, but for only 33" tires, you don't need anything deeper. My 90 k2500 had a th400 and 245/75R16s. It got 8.4mpgs on my first tank. I eventually got it up to about 13.5mpg with no mods to the stock computer. My 93 k2500 had a best tank ever of 14.25mpg with its 4L80E trans. Both trucks have 4.10s. I eventually went with a holley sniper system on the 90. But it was shot in the foot from the start with the th400 trans. Highway rpms were ridiculous, with 2400rpms being 50mph. Passing on the interstate was sometimes a 4000rpm affair.. and that's sustained. I eventually installed a 4L80E and I'm much happier with the cruise rpms. Calculate your cruise rpms with 5.38s and see what your rpms will be at 75 or 80.

With all due respect I did not recommend 5.38 gears.

I recommended a gear that would keep him @ 2300 to 2500 rpm at his cruising speed on the highway.

Before the OP buys any gears I say this: Drive on the highway in drive not overdrive as a test, with a vacuum gauge set so you can see it as you drive.
Drive is the same as overdrive with 4.88:1 gears. See if that gives a higher vacuum signal.

If you can drive the same road speed with higher vacuum you can get better mileage.

Also the computer will not go into lean cruise if the vacuum is too low.
Low vacuum tells the computer that you are under a heavy load and it adds more fuel to make more power.

Getting the tools and taking the time to tune the computer well will absolutely help.

 

[stutaeng]

I run 40-45 psi on LT265/75/16 on my 1500 2wd. 50psi may be a bit on the high side.

The 2500/3500s require like 50 front/60 or 80 psi on the rear, depending on actual vehicle; that's due to added weight and higher payload.

I'll have to ask my brother what MPG he used to get on his 350 TBI RCSB 1500 2wd. I want to say around town, not too good, maybe 12-13? His 700R4 is not going into OD, and he's just running it like that (I keep joking that he might as well swap a TH400!!! ) so highway mpg are, who knows???..He regeared to 3.73, and has some tires around 30" diameter? I don't really know the size...

What were these trucks rated for by the EPA anyways? And what are folks getting on stock-ish trucks? That's probably what OP needs as a baseline...

 

[Frank Enstein]

I disagree wholeheartedly with the guy who recommended 5.38s for your 33" tires. Gm designed these engines to get better mileage by turning slower. As long as you shift into overdrive and your torque converter clutch locks up, your gears are fine. Maybe a tad tall, but not ridiculously so.. since you're only running a couple inches taller than stock.
The roof rack and bumper and winch all add both weight and wind resistance.
I think the high idle is telling. Does your truck smell like raw gasoline at idle? Is it hard to start when hot? I think your computer isn't seeing your coolant temp sensor. I had a bent connector on my most recent tbi truck. It plugged in but didn't quite touch the pins on the coolant temp sensor. The computer thought the coolant was -40⁰ and was absolutely dumping gas in on the coolant temp enrichment circuit. That would also increaae your idle speed like a choke in older engines. As the engine hears up, the idle should drop to about 625rpm. The gauge and computer use different sensors, so your gauge may read fine. Another factor may be your thermostat.. I recently changed back to a 195⁰ thermostat. I thought a 180⁰ would help performance but the computer doesn't really like it that much. It only keeps the engine from going into lean cruise mode because the computer thinks it's it's not fully up to temp.
The torque converter clutch won't lock if the coolant temp is too low.. so if the coolant temp sensor isn't reading correctly, the tcc won't lock and you'll be using more fuel.
If I'm going for maximum mileage, I air the tires up to max pressure or even a few psi over. I've never heard of dividing the weight by the max load and dividing by the max pressure before. That assumes that the load capacity increases linearly with pressure. And an empty truck isnt that heavy on the rear so you'll be underinflating the tires... Isn't that how Ford Explorers were blowing all of those tires in the 90s?
The stock tbi timing table is extremely conservative. I added between 4⁰ and 22⁰ spark advance to my 92 c1500's tables. Wide open throttle was set for 16.34⁰. That's incredibly inefficient. These trucks typically like 34⁰ of total timing. Unless you can get a custom chip or tune the timing tables yourself, I'd set your base timing to 6⁰ instead of 0⁰ and see how it goes. If you hear pinging, back it off. Proper spark timing has a huge benefit for both power and economy. I tuned mine over the course of a couple weeks, datalogging the drive back and forth to work, then making small tweaks. If I ever saw activity on the knock sensor, id immediately pull 4⁰ from that cell.
My truck has headers and free-flowing exhaust. That's the only engine modification. It's a 92 c1500 rclb. 350tbi. 700r4. 3.42. 235/75r15s. I tuned the tbi computer with my laptop. I'm getting pretty good mileage with mine once I've got everything dialed in. Gm should have tuned them like this from the factory.
As far as gearing, you can go with 4.10s, but for only 33" tires, you don't need anything deeper. My 90 k2500 had a th400 and 245/75R16s. It got 8.4mpgs on my first tank. I eventually got it up to about 13.5mpg with no mods to the stock computer. My 93 k2500 had a best tank ever of 14.25mpg with its 4L80E trans. Both trucks have 4.10s. I eventually went with a holley sniper system on the 90. But it was shot in the foot from the start with the th400 trans. Highway rpms were ridiculous, with 2400rpms being 50mph. Passing on the interstate was sometimes a 4000rpm affair.. and that's sustained. I eventually installed a 4L80E and I'm much happier with the cruise rpms. Calculate your cruise rpms with 5.38s and see what your rpms will be at 75 or 80.

The Ferd Exploder/Firestone debacle was caused by Ferd insisting on a lower tire pressure (28 psi) for ride quality and people not checking the tire pressure. Bridgestone/Firestone didn't want the pressure that low. In many cases the remaining tires on the vehicles were found to be as low as 20 psi.


And that is why we have to have TPMS on any new car.

Ferd didn't care if it killed people.

Remember the Pinto? It was cheaper to settle out of court for people dying than to put a $10 piece of plastic on the front of the tank on a million Pintos.

 

[L31MaxExpress]

With all due respect I did not recommend 5.38 gears.

I recommended a gear that would keep him @ 2300 to 2500 rpm at his cruising speed on the highway.

Before the OP buys any gears I say this: Drive on the highway in drive not overdrive as a test, with a vacuum gauge set so you can see it as you drive.
Drive is the same as overdrive with 4.88:1 gears. See if that gives a higher vacuum signal.

If you can drive the same road speed with higher vacuum you can get better mileage.

Also the computer will not go into lean cruise if the vacuum is too low.
Low vacuum tells the computer that you are under a heavy load and it adds more fuel to make more power.

Getting the tools and taking the time to tune the computer well will absolutely help.

/\/\/\ When I had a stock 5.7L Vortec, 4L60E and 3.42s in my 97 Express running at 75 mph there was zero difference between 3rd and 4th in MPG but a hell of alot more power in 3rd. Vortec makes more power everywhere than a stock TBI. It was not until I put tri-y headers, a 4L85E with a shallower overdrive ratio and 3.73s in the back that overdrive was truly a useable gear. I went from 1,950 rpm at 70 mph to 2,400 rpm at 70 and gained MPG. The 97 has 32" tall tires and a 5.13 gear now. With the 383, tows my travel trailer down the highway in overdrive.

 

[Hipster]

I went from 1,950 rpm at 70 mph to 2,400 rpm at 70 and gained MPG. The 97 has 32" tall tires and a 5.13 gear now. With the 383, tows my travel trailer down the highway in overdrive.

^^agree.

There's a big misconception that low rpm= ecomony. Any engine will achieve better economy cruising at or closer to it's torque peak for a given load. It doesn't matter, boats, generators, briggs and stratton, and other engine driven equipment. Brake specific fuel consumption numbers Vs. torque output are on a dyno sheet but never hardly looked at. Maintaining a given torque value at too low an rpm generally requires more throttle plate opening. Under-geared is counter productive to mpg's.

Op, under-inflated tires don't help and how you leave from a stop light is a big contributing factor as is driving 65 vs. 80. Drive like G'ma.

Mine has 33's and 3.73's and that's barely adequate IMHO. Too much throttle action in OD. I get the same/negligible difference in mileage towing 4000k in 3rd turning 26-2800 rpm steady state as towing empty trailer in od turning 1800-2k on routine 1000 mi trips. Around town I get maybe 11-12mpg with a tailwind.

 

[1989GMCSIERRA]

I can't see the tires tanking fuel economy that badly. I run the same size but with more gear and an assload more torque. Something else is wrong here. 3.42 and a 33 is a lot, but not the cause for single digit fuel economy. That's worse than my supercharged big block dually gets.

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.

 

[Martin Evans]

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!

"My EGR vacuum line from the solenoid to valve also broke and I've yet to replace it"
That's a good part of your problem. Fix the vacuum leak and many of problems will also be corrected. Also, gap your spark plugs.

 

[PlayingWithTBI]

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

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