Square Body Suburban Info

[nitro_rat]

I routinely got 17 hwy in my 88 1/2 ton 'Burb. TBI 350 tired but in good tune, base timing advanced 10°, 93 octane gas, no lift, 3.42's, no lift, 33x12.50 Nitto Terra Grapplers until that size was discontinued, 33x12.50 Buckshots after that until its untimely demise. 13-15 around town. Gas was probably a little better back then, those were the days before 10% ethanol...

Yes, the speedometer was corrected!

 

[Oilbrnr]

Yes, the speedometer was corrected!

Yeah, but was your odometer corrected?

Having been around Squares my entire life, owning several, still do, I call BS.

 

[SexyExy]

A stiff headwind can easily mean 3 to 4 mpg reduction, especially at speeds over 70 mph. The newest truck in my fleet is a 2016 Ford F350 4x4 with the 6.7 Powerstroke. Completely stock except for a custom camper shell that is 8" above the cab. Empty highway mileage is strictly speed and headwind related. Drove from Sacramento to Vegas for the SEMA show....got right around 14.5 mpg (never went below 70 mph on the freeway, usually between 75 and 80 mph). Coming back on the exact same route I got just over 16 mpg. Only difference was the headwind on the way to Vegas.

Towing a 24' enclosed race car trailer with a couple of AC type units on the roof, I get around 10 mpg when never going below 70 mph, usually cruising between 70 and 75 mph. The digital mpg display is usually within a tenth of my hand calculated math so it is dead nuts accurate (unfortunately, lol). I've towed the same route multiple times with the trailer empty and with a 3700 lb car in it....towing mileage is the same when towing flatland. The aerodynamic drag from the trailer just kills the mileage. In a severe headwind (20 plus mph constant winds) I've gotten as low as 8.6 mpg when towing the enclosed trailer. I also have an all steel deck heavy duty flatbed trailer that combined with the truck weighs nearly 14k pounds when empty. When towing the flatbed trailer on the highway at 60 mph I get 14.5 mpg....weight is not the issue unless in a lot of hills, etc. We won't even discuss stop and go driving when talking about trucks this size....they all suck when it comes to mileage.

Cruising in my 91 Sub with bone stock size tires I could get an easy 17 mpg at 60 mph. My lifted 85 with 5.13 gears, 35" tires and a 700R overdrive would get about 12 mpg at those same speeds. Stop and go driving was single digits in both trucks....not even worth keeping track of.

If you really want to know the fuel mileage of a vehicle, fill it up and note the mileage. Drive it for 60 days while noting the gallons used during that time. Divide the total gallons used into the miles driven for 60 days. That is your real world mileage. Taking a single mileage reading from one trip or couple of tanks of fuel is statistically meaningless.

 

[zoomad75]

I'm a nerd for numbers when it comes to my truck. Granted, it's a K5 Blazer vs a Suburban but it's tipping the scales closer to a Burb now with the camper and gear inside. I can totally attest to aero and headwind having a major effect on mileage in my square. My K5 is running a 5.3L Vortec LS swap, 700r4 trans with 4.10 gears in the D44 and 14b axles and 35" tires. Prior to the camper install, I was routinely averaging 15-16mpg on the highway over multiple tankfuls. Post camper install the average dropped to 11-12 depending on wind and terrain. The camper's overhang and straight vertical front wall that sticks out from the sides are like pushing a barn door down the highway.

I track every tank through an app on my phone that takes the odometer reading when I fill up and the amount of fuel I put in. It subtracts the mileage from the previous fillup and then divides it by the fuel amount I plug in. This is all assuming the odometer on the truck is accurate. In my case having the electric speedometer the DRAC module was recalibrated by me to account for the gearing change and tire size. I've compared it to two different GPS sources and it's right on the money for speedometer and odometer values.

The main loss is aero drag for sure. But as I've found the heavy K5 is weak on the mountain climbs. The poor little 5.3 is out of it's torque band unless the trans is all the way down to 2nd gear and the engine is spun up to 4,500 rpm or more. Sure gearing up to a 4.56 ratio might help, but it just illustrates the lack of low end the 5.3 has when pushing a heavy truck up hill. Circling back to the OP looking for insight on a squarebody burb, these are heavy, un-aerodynamic bricks to push down the road. Yes, it's similar to the JK Jeep in that it has a solid front axle, but unlike the Jeep it weighs in 1500 to 2000 pounds more. We can't fight physics with one of these. It takes power move them down the road and it's going to come at a cost of fuel economy. The more you lift and larger tires increases your frontal area and therefore drag. So the balance comes from trading off lift and tire size for reasonable off road ability and on road mileage. Such are the joys of playing with old trucks.

 

[BCobe]

Do some digging and you'll find my buddy's 89 3/4 ton Burb nicknamed Polar Bear.

LS engines are another option, but stick to 6.0L or later 6.2's to maximize low end torque. They won't net much better fuel economy though.

Actually read that entire build thread yesterday. I was really just looking at MPG for research purposes. Compare and contrast with other potential platforms.

The 6.0 LS swap was an idea I was actually looking at. I have 2 other on going bigger projects so not trying to dive that deep of the bat.

The next idea was a Wagoneer but those seem to be gold now or another WJ like my old one. Even that with 6.5 lift, 35s, and the 4.7 got 9mpg highway.

I have a newer F250 so if I go the Burb route it will not be used much for towing.

 

[nitro_rat]

Yeah, but was your odometer corrected?

Having been around Squares my entire life, owning several, still do, I call BS.

The odometer is driven by the speedometer, so...

 

[nitro_rat]

Sure gearing up to a 4.56 ratio might help, but it just illustrates the lack of low end the 5.3 has when pushing a heavy truck up hill.

I have 4.10's in my '01 Tahoe with factory 30" tires. It's the only 5.3 truck I've ever had that wasn't gutless. You'd have to go to 4.88's to get the same final drive with 35's...

 

[ClovisMan]

I can honestly say I have never calculated MPG in my 91. It's not why I bought it.

 

[OneL]

I have a bone stock '78 K20 Suburban with a 400 small block, Quadrajet, Turbo 400, NP203 full-time 4wd and 4.10 gears. I average 8-10 mpg. Thought about an EFI/4L80E/NP205 swap but the time and expense involved will buy a fair amount of fuel so I decided to enjoy it for what it is for now. All around great expedition vehicle for the types of activities I'm involved with. If I were to give any recs it would be to look for a later Square with an OD trans and EFI setup.

 

[zoomad75]

The odometer is driven by the speedometer, so...

Not many go searching for the right speedometer gear to correct for cable driven speedos when gearing and tires sizes get changed on these trucks. More common to have an error for larger tires or gear ratio changes. 90-91 trucks it's pretty easy like mine. Crack open the DRAC module, do the math and look it up on the web for a specific number based on the tire and axle ratio numbers and change the jumpers accordingly. Pretty easy to do and really accurate if you are accurate with the tire size/diameter.

I have 4.10's in my '01 Tahoe with factory 30" tires. It's the only 5.3 truck I've ever had that wasn't gutless. You'd have to go to 4.88's to get the same final drive with 35's...

Totally agree. Add to that the deeper gear ratio negating any possible gains in fuel economy due to the consistently higher cruise rpm on flat ground. Not that building a squarebody is the direction to go for a decent fuel mileage off-road toy. But I've been on the other side of the coin too. My old 75 K5 had a well worn 350 with a Q-jet, 465 manual trans, 3.73 gears in 1/2 ton axles and 35" tires. Speedo was off due to the tires so it read slow. But I recorded a few tanks using GPS to track the actual miles. 8-9 mpg was the norm. That was a normally empty truck, not loaded for a weekend or a week's worth of off-road fun either. So I'm trying to strike a balance between having the power the truck needs to drive on the highway and mountain passes and getting halfway decent mileage. The 8.1 in my plans seems odd to think I'd get better mileage but comparing notes with Larry and his K10/camper weighing in 1500 to 2000 pounds more than mine with his running 4.56 gears and mine running 4.10's might get a tad better than his. He's seeing 11-12 now loaded which I'm pretty close to as is with the 5.3. But even if my mileage stays around the same number, I'll have the power to maintain a safe speed up grades without spinning the engine to 5,000 RPMs and higher. For me, having the power to keep up with traffic is the main goal, the possibility of incremental gains in economy is a bonus.

I can honestly say I have never calculated MPG in my 91. It's not why I bought it.

It's insane that I do. But I'm crazy that way. I like knowing. Then again if I was really stressing on needing tremendously better fuel economy I'd use a better vehicle that is just as capable, smaller and more fuel-efficient. I just like keeping track.

I have a bone stock '78 K20 Suburban with a 400 small block, Quadrajet, Turbo 400, NP203 full-time 4wd and 4.10 gears. I average 8-10 mpg. Thought about an EFI/4L80E/NP205 swap but the time and expense involved will buy a fair amount of fuel so I decided to enjoy it for what it is for now. All around great expedition vehicle for the types of activities I'm involved with. If I were to give any recs it would be to look for a later Square with an OD trans and EFI setup.

Nothing wrong with using what works. I felt that way with my old 75. The 91 came my way without an engine, I had the 5.3 from a customer and time to acquire the stuff to put them together. I can't say it was cheap though, even with a lot of horsetrading on some parts. It's not monumental enough to say the swap will pay for itself in fuel savings that's for sure. But better driveability, self adjustability for elevation and temperature and consistent easy starting have been worth it.

 

[jonathon]

I could see a V1500 with a 700R4 returning 17 if it was in tune on stock tires. My old V2500 could easily return 15 when it was stock on 265/75R16’s and that’s with a 4L80E and 3.73’s. 4” lift, 35’s, and 4.88’s and it was down to 12 at best. It was an awesome truck and I regret selling it.

With how old these trucks are getting I wouldn’t put a whole lot of concern into the condition of the stock drivetrain so long as the body and frame are in good condition. You want barn doors, the tailgates on these suck big time as they always rust even in salt free environments and the windows get stuck. The only other place of concern for me is the frame around the steering box. Probably not an issue on a clean stock truck, but lifted on big tires watch out for frame cracks. Plan on an engine and trans swap unless you find a true survivor.

Regarding the GMT400; GM improved the creature comforts substantially but in my opinion they do not age as well, especially the interiors.

 

[billiebob]

It's been 30 years since I had a sub, it was a 3/4 ton with bard doors, a 350 automatic. Never got 15mpg. gas mileage will go to single digits as soon as you add a lift and more rubber. The best combination is box stock with 235/85R16s. Parts are plentiful everywhere. As are after market parts. Every second garage sale will be selling something square body.

The big enemy on a suburban is adding aerodynamic aids outside. The thing is already a brick. A roof rack, rtt is like adding air brakes. But you can't find a better family expedition vehicle.

 

[nitro_rat]

Not many go searching for the right speedometer gear to correct for cable driven speedos when gearing and tires sizes get changed on these trucks. More common to have an error for larger tires or gear ratio changes. 90-91 trucks it's pretty easy like mine. Crack open the DRAC module, do the math and look it up on the web for a specific number based on the tire and axle ratio numbers and change the jumpers accordingly. Pretty easy to do and really accurate if you are accurate with the tire size/diameter.



Totally agree. Add to that the deeper gear ratio negating any possible gains in fuel economy due to the consistently higher cruise rpm on flat ground. Not that building a squarebody is the direction to go for a decent fuel mileage off-road toy. But I've been on the other side of the coin too. My old 75 K5 had a well worn 350 with a Q-jet, 465 manual trans, 3.73 gears in 1/2 ton axles and 35" tires. Speedo was off due to the tires so it read slow. But I recorded a few tanks using GPS to track the actual miles. 8-9 mpg was the norm. That was a normally empty truck, not loaded for a weekend or a week's worth of off-road fun either. So I'm trying to strike a balance between having the power the truck needs to drive on the highway and mountain passes and getting halfway decent mileage. The 8.1 in my plans seems odd to think I'd get better mileage but comparing notes with Larry and his K10/camper weighing in 1500 to 2000 pounds more than mine with his running 4.56 gears and mine running 4.10's might get a tad better than his. He's seeing 11-12 now loaded which I'm pretty close to as is with the 5.3. But even if my mileage stays around the same number, I'll have the power to maintain a safe speed up grades without spinning the engine to 5,000 RPMs and higher. For me, having the power to keep up with traffic is the main goal, the possibility of incremental gains in economy is a bonus.


It's insane that I do. But I'm crazy that way. I like knowing. Then again if I was really stressing on needing tremendously better fuel economy I'd use a better vehicle that is just as capable, smaller and more fuel-efficient. I just like keeping track.


Nothing wrong with using what works. I felt that way with my old 75. The 91 came my way without an engine, I had the 5.3 from a customer and time to acquire the stuff to put them together. I can't say it was cheap though, even with a lot of horsetrading on some parts. It's not monumental enough to say the swap will pay for itself in fuel savings that's for sure. But better driveability, self adjustability for elevation and temperature and consistent easy starting have been worth it.

There's a chart for the speedometer gears just like there's a chart for the DRAC module pin outs. Real simple as long as you only need to change the driven gear. Not much more complicated if you need to swap the drive gear too. If you're like me and have owned/driven/parted out these trucks for years you just go get the right color gear out of the pile...

You'll likely see a gain in economy going to a lower (higher numerically) gear in your 5.3 K5. If you keep the RPM's at or just below 2000 when cruising on the highway (probably just below 70 mph) you will probably do ok. Even better if you drive 65!

I could see a V1500 with a 700R4 returning 17 if it was in tune on stock tires. My old V2500 could easily return 15 when it was stock on 265/75R16’s and that’s with a 4L80E and 3.73’s. 4” lift, 35’s, and 4.88’s and it was down to 12 at best. It was an awesome truck and I regret selling it.

With how old these trucks are getting I wouldn’t put a whole lot of concern into the condition of the stock drivetrain so long as the body and frame are in good condition. You want barn doors, the tailgates on these suck big time as they always rust even in salt free environments and the windows get stuck. The only other place of concern for me is the frame around the steering box. Probably not an issue on a clean stock truck, but lifted on big tires watch out for frame cracks. Plan on an engine and trans swap unless you find a true survivor.

Regarding the GMT400; GM improved the creature comforts substantially but in my opinion they do not age as well, especially the interiors.

Yes, a V1500 in stock condition will return 17 mpg. 3.73's with 31x10.50's was the best stock combination. With a few mods it can be improved a little. I found the 3.42/33's combo to work well for economy but I would have prefered a lower (higher numerically) gear. 100% agree on the barn doors. The tailgates suck especially if they're electric.

It's been 30 years since I had a sub, it was a 3/4 ton with bard doors, a 350 automatic. Never got 15mpg. gas mileage will go to single digits as soon as you add a lift and more rubber. The best combination is box stock with 235/85R16s. Parts are plentiful everywhere. As are after market parts. Every second garage sale will be selling something square body.

The big enemy on a suburban is adding aerodynamic aids outside. The thing is already a brick. A roof rack, rtt is like adding air brakes. But you can't find a better family expedition vehicle.

Any non-overdrive squarebody will be a gas pig. I agree that stock gives the best results in MOST situations.

 

[BCobe]

I could see a V1500 with a 700R4 returning 17 if it was in tune on stock tires. My old V2500 could easily return 15 when it was stock on 265/75R16’s and that’s with a 4L80E and 3.73’s. 4” lift, 35’s, and 4.88’s and it was down to 12 at best. It was an awesome truck and I regret selling it.

With how old these trucks are getting I wouldn’t put a whole lot of concern into the condition of the stock drivetrain so long as the body and frame are in good condition. You want barn doors, the tailgates on these suck big time as they always rust even in salt free environments and the windows get stuck. The only other place of concern for me is the frame around the steering box. Probably not an issue on a clean stock truck, but lifted on big tires watch out for frame cracks. Plan on an engine and trans swap unless you find a true survivor.

Regarding the GMT400; GM improved the creature comforts substantially but in my opinion they do not age as well, especially the interiors.

If I go the burb route, then I will deffenitly get barn doors. My parents had one of these growing up. I do remember how heavy that thing was. I thought about the GMT400 but I really have no experience with IFS suspensions and pricing a solid axle swap, I was at 4k without the axle.

It's been 30 years since I had a sub, it was a 3/4 ton with bard doors, a 350 automatic. Never got 15mpg. gas mileage will go to single digits as soon as you add a lift and more rubber. The best combination is box stock with 235/85R16s. Parts are plentiful everywhere. As are after market parts. Every second garage sale will be selling something square body.

The big enemy on a suburban is adding aerodynamic aids outside. The thing is already a brick. A roof rack, rtt is like adding air brakes. But you can't find a better family expedition vehicle.

Kind of why I threw this platform into the mix. Sheet metal seems to be dirt cheap on LMC compared to classic car metal, 350's are plentiful and I have 3 of them currently in my fleet, so the cross-compatibility of some parts would be nice.

Weird question does anyone know the dimensions of the rear cargo area without the third-row seat? I will not have access to actually check one out for a few months so doing my comparing and contrasting via numbers and feedback as of now.

 

[NatersXJ6]

You will give up a lot of modern safety, might not matter to you, but think about mama and her opinions if you are hauling littles around.

You also need to be okay with never making a u-turn in your life. That seemed alright to me until I moved to a city that had large numbers of median divided streets. A quest for tighter turns led me to crossover steering and that is really difficult or expensive on a 10-bolt front end. Somewhat easier if you get the right Dana 44 and own a machine shop

On the good side, almost every seal, screw, and clip are available from LMC Truck Catalog, or they were 10 years ago when I had my last squarebody Chevy.

If I was looking for similar but newer, I would look to the Excursion lineup.