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.
