Nobody warned me about the MPGs on these square bodies!

[HillyardSquare]

82 shortbox stepper with a mild 350, Tremec 5 speed and 3.08 gears. On the return trip from Seattle to Spokane, about 275 miles, I got 15.3 mpg. That's at 70 mph the whole way back.
I do plan on 3.73's soon though.

 

[CheemsK1500]

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.

 

[CheemsK1500]

About the same as a 5.3 gmt 800 4x4 pickup or suv. I think the ‘Hoes we’re in the 13s

Yesterday I did a 50 mile highway trip in my 04 2500HD 4x4 with 6.0,4L80, and 4:10 gears and the truck’s mpg estimator read 14.2 MPG. When I drive strictly around town, it dances around 12 MPG.

As for fuel injection conversions on squares and other older vehicles, like others said don’t ever do them just for MPG increases; you will likely be disappointed. A well tuned Q Jet is not going to do substantially worse on a small block than a TBI setup will in strict regard to fuel economy.

 

[Grit dog]

Yesterday I did a 50 mile highway trip in my 04 2500HD 4x4 with 6.0,4L80, and 4:10 gears and the truck’s mpg estimator read 14.2 MPG. When I drive strictly around town, it dances around 12 MPG.

As for fuel injection conversions on squares and other older vehicles, like others said don’t ever do them just for MPG increases; you will likely be disappointed. A well tuned Q Jet is not going to do substantially worse on a small block than a TBI setup will in strict regard to fuel economy.

Agreed on old efi and 4speed slushies vs a good running qjet engine. And agree the roi on an efi conversion isn’t there in most all cases.
BUT apples to apples there’s still a big difference.
If you can get 14 in your old 6.0 and 12 (hahahaha maybe) in an 80s big block K20, all bone stock and running like the day it left Detroit, you got got about a 25% increase in hp small or big block OD so you can use deeper gears and still drive highway. And a bit better with weather and altitude compensation.
Plus define 50 mile “highway” drive. I can’t remember the last time I drove a GMT800 6.0 but the 900s and newer I’ve never seen more than 12 on the highway. Highway here though is mountains and/or 80mph + avg.

 

[Grit dog]

There have been constant mpg improvements over the years and apples to apples again they’re even greater than they look on paper.
The same drive that is 12-13 mpg in my 180 hp clean running C10 is 18mpg in my 400hp crew cab 4wd F150.
Compared to 7-8mpg in the 300+ hp K20 big block. (Not fair because it has 130lb tire/wheel combos and a lift )

 

[CheemsK1500]

the “50 mile highway trip” referenced earlier was with no hills/mountains, and I never went above 75 mph, some sections of the drive, the limit is capped at 65 mph. Also keep in mind, I’m reading the the truck’s built in Avg MPG counter and not doing the math myself. I may do the math myself on the next trip and see how close it is.

 

[Frankenchevy]

When I installed the FITech EFI system on my 79, I had hoped to maybe get a bit better mileage from the truck. Noope. It got 11-12 from the start and that's about what it does now.
Drivability did improve dramatically so it was worth the effort, but it will never pay for itself at the pump.

It’ll take quite a long time (if ever) with FITech, as it’s TBI. A sequentially fired MPFI like the proflo4 will get a little better fuel economy because each firing event is getting a more carefully measured and better atomized shot of fuel.

Where a TBI would theoretically eke out an edge, is adjusting for variables. A “perfectly” tuned carb is only good for a couple thousand feet and a range of temperatures, though the latter may have less effect.

 

[HotRodPC]

With a confirmed accurate speedo and odo, I was able to get 13.2 in a C20 with a 454. Nope, not one of those guys either. I couldn't beleive it myself. I was going easy on it trying to see if I could reach 10mpg and I got 13.2. Now if I'm just driving it, ehhh it gets 8 or 9 depending on how many yellow lights I hit, or how many times I hit a freeway on ramp. That's when you can justify opening up the secondaries. Have to do that to make sure the secondary throttle shaft don't seize up from lack of use.... RIGHT? Yep, RIGHT !!!

 

[Redfish]

For my 120 mile drive from home to the Mississippi Gulf Coast I got 12.5 mpg based on the odometer reading. I know the odometer is no longer "accurate" because of the larger tires so if I had set the GPS it probably would have been a little over 13 mpg. I had hoped to do a little better than that but it is what it is. I drove the old truck pretty much the same way I always drive it. Do I hold first gear a little longer than I should because I like hearing the exhaust rumble? Of course I do. That's part of why I wanted this old truck in the first place.

So...
1987 TBI 350, 3.42 gears, 33x12.50 mud grips, 4 speed manual. 12.5 mpg.

 

[bucket]

With a confirmed accurate speedo and odo, I was able to get 13.2 in a C20 with a 454. Nope, not one of those guys either. I couldn't beleive it myself. I was going easy on it trying to see if I could reach 10mpg and I got 13.2. Now if I'm just driving it, ehhh it gets 8 or 9 depending on how many yellow lights I hit, or how many times I hit a freeway on ramp. That's when you can justify opening up the secondaries. Have to do that to make sure the secondary throttle shaft don't seize up from lack of use.... RIGHT? Yep, RIGHT !!!

Got similar results from our old 454 C20 Burb. Around town in cold weather, it would get 8 to 9 mpg. Warmer weather was 9 or 10 mpg. Rural driving would get 11 mpg. It was an interstate cruiser though as it would get 13+ all day long at 75+ mph.

 

[Radiohead]

Strongly considering a switch from 4.10 to 3.42 here. For highway economy mostly.

 

[bucket]

Strongly considering a switch from 4.10 to 3.42 here. For highway economy mostly.

My pair of C20 Burbs came with 3:73. 4:10 would have been ideal I think, with not much effect on mpg.

 

[Terlingueno]

I wouldn't change my 4.10's and SM465 for anything else...

 

[Radiohead]

I love the 4.10 too, but I figured different gears was less wallet invasive than a modern transmission

 

[bucket]

I love the 4.10 too, but I figured different gears was less wallet invasive than a modern transmission

Yeah that's true, but how many years of driving will it take to recoup your cost with fuel savings?