Nobody warned me about the MPGs on these square bodies!

DoubleDingo · Oct 21, 2025

legopnuematic said:
With a more or less stock driveline I’m not sure you’d see much for gains (strictly mpg, not cost of regearing, etc). 3.42 to 4.10 (same tire size) you’d be about 550 rpm difference.

There are tons of factors, both vehicle and environmental that contribute to fuel economy, far beyond engine rpm at cruise. Lower rpm doesn’t necessarily mean better mpg.

Yep, making sure the engine is at its maximum vacuum at the desired cruise rpms means it's in the sweet spot of the powerband. That is achieved by having the correct engine internals, exhaust pipe size, fuel delivery, etc.

I have a cowroker that has an 81 stepside. It was a single exhaust truck, and he changed it to 3 inch duals. The fuel economy tanked on it, and he couldn't get it to pass emissions. Had to do a bunch of back and forth with the referee to finally get it to pass emissions, but the fuel economy is still way less than it was with single exhaust.

bucket · Oct 21, 2025

Turbo4whl said:
8 pages of fuel mileage. My question, are you all using an app to calculate miles traveled? Back in the day fuel number could be all wrong if the speedo was not re-geared. @Soundmound 's use of 20" tires is a good example how many mods these trucks can receive.

Skimming through these 8 pages @HotRodPC is the only reference I spotted confirming correct miles traveled.

Just to put it out there, my post here with MPG figures stated were figures based on actual distance traveled. Along with any other MPG posts I may have made here over the years.

HotRodPC · Oct 21, 2025

Turbo4whl said:
8 pages of fuel mileage. My question, are you all using an app to calculate miles traveled? Back in the day fuel number could be all wrong if the speedo was not re-geared. @Soundmound 's use of 20" tires is a good example how many mods these trucks can receive.

Skimming through these 8 pages @HotRodPC is the only reference I spotted confirming correct miles traveled.

Exactly right. I hear people saying, I get 19 mpg in my C30 with a 454. BULLLLLLLLLLLLLLLLLLSHHHHHHHIT !!! Just gonna say it. No they are NOT. That's why my post i specified, with an accurate speedo and odo. I checked them. Using mile markers on the freeway for 10 miles, every spot was on point with my odo down to the 10th of a mile, and I figured the freeway markers could be wrong, but naww, they really do seem to be quite accurate within maybe a foot or 2. Then the speedo was checked against a GPS and DigiHUD phone app that is a speedo app using sattelites. I'll assure you, my speedo and odo was 100% accurate. When the mile markers, GPS and Sat phone app all say the same thing, and the DigiHUD app also has ODO too, so when they all agree, I feel it's safe to say it's accurate.

Also I should mention, that 13.2mpg was I was trying hard for the best case and driving accordingly. Soft on the gas pedal, 65mph etc. And I did that for about a 14 gallon run and I checked it. Then I didn't care about my driving and just drove. Checked again, I got like 9. Also, that 13.2 was with 3.21 rear gears and 95% highway driving. Now the truck has 3.42 rear gears, speedo is now off of course, but I'll be it'll not get that 13.2 now. It might, noway to tell really unless maybe I used the DigiHud App for a long drive all at the same time then fill back up.

But yeah peeps, check your speedo and odo for accuracy before saying for SURE what your mpg is. A re gear in the read end can make it inaccurate. Worn speedo gears can throw it off, a trans swap that has different speedo drive and driven gears can throw it off. Some of you might be getting better than you think, and some of you are not getting what you think.

bucket · Oct 21, 2025

HotRodPC said:
Exactly right. I hear people saying, I get 19 mpg in my C30 with a 454. BULLLLLLLLLLLLLLLLLLSHHHHHHHIT !!! Just gonna say it. No they are NOT. That's why my post i specified, with an accurate speedo and odo. I checked them. Using mile markers on the freeway for 10 miles, every spot was on point with my odo down to the 10th of a mile, and I figured the freeway markers could be wrong, but naww, they really do seem to be quite accurate within maybe a foot or 2. Then the speedo was checked against a GPS and DigiHUD phone app that is a speedo app using sattelites. I'll assure you, my speedo and odo was 100% accurate. When the mile markers, GPS and Sat phone app all say the same thing, and the DigiHUD app also has ODO too, so when they all agree, I feel it's safe to say it's accurate.

Also I should mention, that 13.2mpg was I was trying hard for the best case and driving accordingly. Soft on the gas pedal, 65mph etc. And I did that for about a 14 gallon run and I checked it. Then I didn't care about my driving and just drove. Checked again, I got like 9. Also, that 13.2 was with 3.21 rear gears and 95% highway driving. Now the truck has 3.42 rear gears, speedo is now off of course, but I'll be it'll not get that 13.2 now. It might, noway to tell really unless maybe I used the DigiHud App for a long drive all at the same time then fill back up.

But yeah peeps, check your speedo and odo for accuracy before saying for SURE what your mpg is. A re gear in the read end can make it inaccurate. Worn speedo gears can throw it off, a trans swap that has different speedo drive and driven gears can throw it off. Some of you might be getting better than you think, and some of you are not getting what you think.

Alright @HotRodPC I'm calling you out, lol. Who in the he|| claims to get 19 mpg in a 454 anything?! Usually they b|tch that they only get 4 mpg and use it as an excuse to swap in a turbo diesel that gets 35 mpg while pulling 20k! Are you drunk? :rofl:

Turbo4whl · Oct 21, 2025

HotRodPC said:
Exactly right.

Thank you! I was happy when I could, once in a while, squeeze double digits out of the 402 Rat.

Hunter79764 · Oct 22, 2025

CheemsK1500 said:
I wonder how the folks the 292 and 4.3 equipped trucks do mpg wise. I don’t see too many V6 or L6 squares out in the wild. I don’t know if they were just bad sellers back in the day, or everyone just did a V8 swap at the soonest opportunity given.

For an Apples and Bananas comparison, I have an 85 Monte Carlo that was stock 4.3 and 3 sp auto and 2.14 rear gear (it basically feels like a 4 speed that is missing 1st gear) that got ~15 city and ~25 highway at 75-85 mph. After a few years, I swapped it to a stock 5.3 (not even tuned beyond removing VATS and eliminating extra sensors and codes) and kept the trans/rear end combo, and city mpg jumped to 19-20, and highway jumped to... 19-20. I was a bit bummed, but it made the "How far can I get on a tank" calculations much easier.
(to be fair, I think the lack of VSS signal and lack of tune hurt it quite a bit, but the transmission died a few years ago and I haven't messed with it since. TH350 cores are surprisingly hard to find for cheap, and if I upgrade then I basically need to dump $5-6k into it to round it out.)

I don't have numbers on MPG for the Suburban, but I'm hoping after the build (8.1/5 speed) to crack 10 mpg at least occasionally. I get 6-7 with an 8.1 in a much taller, wider, longer, and heavier motorhome with a 5 speed Allison turning closer to 2900 rpm @70 mph, this setup should be closer to 2200 highway with a lot less load, so surely it won't be 7 or 8, right? Right?

YakkoWarner · Oct 22, 2025

OK you guys have me questioning my own reality. I know my speedo is off by (on average) +5 or so at highway speeds, which I just attributed to bad springs in the instrument (when it says 70 I'm going around 65 and it isn't always perfectly consistant when compared to GPS). Now I have to do a odometer check - I assumed it was right since its just a straight mechanical gear train.

bucket · Oct 22, 2025

YakkoWarner said:
OK you guys have me questioning my own reality. I know my speedo is off by (on average) +5 or so at highway speeds, which I just attributed to bad springs in the instrument (when it says 70 I'm going around 65 and it isn't always perfectly consistant when compared to GPS). Now I have to do a odometer check - I assumed it was right since its just a straight mechanical gear train.

I've got two different vehicles where the speedo was off a bit, but the odometer was spot-on correct. One mechanical ('78 Suburban) and one electronic ('00 Express).

I've since changed tire sizes and it has worked out that the speedo is correct, but I have to adjust my calculations for mpg now.

Sad Sack · Oct 24, 2025

Last fill up indicated I'm getting 14.4 mpg in town, not fantastic but not bad for a standard cab fleetside. Would probably get better if I retested with a long highway drive.

Keith Seymore · Oct 24, 2025

CheemsK1500 said:
I wonder how the folks the 292 and 4.3 equipped trucks do mpg wise. I don’t see too many V6 or L6 squares out in the wild. I don’t know if they were just bad sellers back in the day, or everyone just did a V8 swap at the soonest opportunity given.

I think the best I did was 26 mpg. Normally around 22 mpg or so.

That's 4.3L TBI, MY6 manual overdrive, 3.73 rear axle and P235/75R15 tires. Truck is a 1/2 swb, lowered about 4 inches, with the 4wd front air dam.

Mine was a daily for 30 years and 200k miles.

K

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Scruffy49 · Jun 1, 2026

Late as usual but…
1985 C10 swb as issued with Qjet on a 4.3, Th400, 4wd airdam, open 2.73s, P205/75r15 on 15x6 rally wheels got 14-16 town, up to 29 hwy. Swapped to P215/70r15s, 14-16 town, 24-26 hwy. Swapped to LT235/75r15D ATs, 11-13 town, 18-20 hwy.

1992 K1500 extended cab short bed, tbi 350, 700r4, NP241C, 3.42s, LT265/75r16E AT, 8mpg combined.

1999 K3500 rc dually, Vortec 350, NV4500, BW4470, 4.10s, LT235/85r16E AT steers LT265/75r16E AT drives, 10.6-11.5 combined.

ALL calculated with the same speedometer/odometer app on an I-phone.

PrairieDrifter · Jun 1, 2026

My mpg was calculated correctly. Just had to clear my name :rofl:

Since I have oversized tires.

Dakota digitals and made sure I let the nozzle shut off itself on the fill before the test and after. No extra squeeze.

DoubleDingo · Jun 1, 2026

Scruffy49 said:
Late as usual but…
1985 C10 swb as issued with Qjet on a 4.3, Th400, 4wd airdam, open 2.73s, P205/75r15 on 15x6 rally wheels got 14-16 town, up to 29 hwy. Swapped to P215/70r15s, 14-16 town, 24-26 hwy. Swapped to LT235/75r15D ATs, 11-13 town, 18-20 hwy.

1992 K1500 extended cab short bed, tbi 350, 700r4, NP241C, 3.42s, LT265/75r16E AT, 8mpg combined.

1999 K3500 rc dually, Vortec 350, NV4500, BW4470, 4.10s, LT235/85r16E AT steers LT265/75r16E AT drives, 10.6-11.5 combined.

ALL calculated with the same speedometer/odometer app on an I-phone.

Says a lot about tire size and gearing combo

Nobody warned me about the MPGs on these square bodies!

DoubleDingo

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