hypothetical, mpg

[keithb]

i have a 1980 c10 shortbox. 250 inline, 700r4, 3.73 14bolt semi float (6 bolt swapping). what would you folks due for a mileage build. i already have an old car with power. the challenge is mpg this time. its going to be lowered. i plan on running a tin canopy. im not stuck on the 700r4. honestly i would love to find an od for the 3 on the tree currently in it. what are your thoughts. i would love to crank up the compression and run propane but im not sure what way that would go. for wheels i would like to run transport wheels and between 28 and 30 inch tires. maybe a stager. what would you folks doo differently, if you care.

 

[jh86]

Personally, I wouldn’t build a Squarebody for gas mileage. I think it’s silly that people want an 80s or earlier truck to get great mileage. You’ll spend more money building it that way then you’ll ever save on gas, so what’s the point? If you don’t drop big money for modern gas mileage features then you’re chasing a few mpg. And you aren’t going to change the poor aerodynamics; which is a huge factor.

 

[squaredeal91]

Ive thought about this myself but haven't done it yet. Build a smaller displacement small block like a 305 for torque power. They even make mileage grind cams i think. Get a 2 barrel dual jet 210 probably taller gears than 3.73,like 3.08s and a 5 speed manual transmission. Should be able to get 20-25 mpg

 

[squaredeal91]

Propane is cheaper but you lose in power. The more torque you have the easier it is to move the brick

 

[ali_c20]

Welcome from Austria. Wide tires are not good for mileage. 3.73 gears are also not great.
A diesel and a 5 speed manual trans would give you best mileage.

 

[jcperformance]

i have a 1980 c10 shortbox. 250 inline, 700r4, 3.73 14bolt semi float (6 bolt swapping). what would you folks due for a mileage build. i already have an old car with power. the challenge is mpg this time. its going to be lowered. i plan on running a tin canopy. im not stuck on the 700r4. honestly i would love to find an od for the 3 on the tree currently in it. what are your thoughts. i would love to crank up the compression and run propane but im not sure what way that would go. for wheels i would like to run transport wheels and between 28 and 30 inch tires. maybe a stager. what would you folks doo differently, if you care.

That's a fun challenge for an old squarebody. Since you're chasing MPGs, the 700r4 is actually a solid choice because of that overdrive, though finding an old Gear Vendors unit for the 3-on-the-tree would be way cooler. Propane is a neat idea for high compression, but keep in mind it usually has lower energy density than gas, so your actual mileage might dip even if it runs cleaner. I'd definitely focus on aero since those trucks are basically bricks, so lowering it and adding that canopy will help more than you think.

 

[Scruffy49]

Before we started restomodding it, Lisa’s truck got 13-16 in town, 16-18 on the freeway, and has turned a best of upper mid-20s on long trips.
4.3 with a factory installed Qjet, factory Th400, factory open 2.73 rear gears. Was on LT235/75r15s all around. Period correct aluminum topper. Supposed to run P205/75 per the spid…
Will stay 4.3, Th400. Getting a locker or mini spool, staying 2.73 but with 30-9.5 rear tires, 215/70s up front. Static 5/7 or 6/8 drop, however it settles with 2.5-3” drop spindles, cut coils, rear flip/C-notch, and f/r shock relocators. Alloy intake, 500 cfm 4bbl, high flow cats and 2 in 1 out muffler.

Will it improve, decrease or stay the same? No idea, it’s 41 years old, and even dropped that far has the aerodynamics of a kitchen stove on a rollerskate…

If you see improvements, consider them a bonus. Good luck with your build, take brakes, try to keep it fun.

 

[TotalyHucked]

So there's 3 things in your statement that will work against your fuel mileage goals, the 14-bolt, the 3OTT and the transport wheels.

The 14-bolt is heavy, a 10-bolt or 12-bolt would be a better choice there.

The 700R4 is going to get better mileage than a 3OTT simply because it has a deeper first gear that will get you moving better but then you have an overdrive with a lockup converter. The ODs that hang off an old 3OTT kinda suck IMO, they're very clunky and also very heavy. 700R4 gets my vote.

The transport wheels (I'm assuming you're referring to the GM 22" transports) are HEAVY. AS. FFFFF. Rolling mass and sprung weight are at odds with a fuel mileage build. You'd be much better off sticking with 15" steelies/rallies or going with a cast aluminum wheel.

We have a bone stock '76 C10 shorty for our shop truck, 235 6cyl, 3OTT (not sure what gear is in it) and the best it's ever done is 15-16mpg. I think with a 700R4, mild drop kit and the 15s with skinny tires, it could get upper teens to 20 with perfect conditions.

 

[Hunter79764]

4BT Cummins and a 5 or 6 speed stick? Otherwise, the straight 6 with decent compression and either a 5 speed or the 700R4 seem like a good route. Agree that the rear end will not help you, even a semi float 14 bolt is fairly beefy, heavy, and power hungry.

 

[goldpack]

what is the purpose for it. ? boulevards and 2 lanes ? poking along at 20-40mph.
(32kph- 64kph)

is it just for fun on weekends? going to put a business in it?

 

[Bextreme04]

i have a 1980 c10 shortbox. 250 inline, 700r4, 3.73 14bolt semi float (6 bolt swapping). what would you folks due for a mileage build. i already have an old car with power. the challenge is mpg this time. its going to be lowered. i plan on running a tin canopy. im not stuck on the 700r4. honestly i would love to find an od for the 3 on the tree currently in it. what are your thoughts. i would love to crank up the compression and run propane but im not sure what way that would go. for wheels i would like to run transport wheels and between 28 and 30 inch tires. maybe a stager. what would you folks doo differently, if you care.

If you are willing to build the engine, I would rework the 250 to have a better head, roller cam, 9.0:1 compression. Then slap a turbo or supercharger on it and run fuel injection with a 4L60e. 10 bolt rear with 3.05 gears from a later model truck(like the one you get the 4l60 from) You could also just keep the 700R4 and run a standalone holley sniper or Fitech. Run low boost and your fuel economy without stomping on it will be great. The inline 6's especially really get a boost in low end torque with a smallish turbo or supercharger. BSFC is greatly improved with turbos at low boost levels and in cruise conditions because it is reusing heat energy that would normally be wasted from the combustion process.

The EFI is gonna be where you get the best repeatable numbers under all conditions. The smaller displacement of the 250 combined with the amount of torque it is gonna make down low(especially turbo'd) will have it running pretty efficiently. Lower RPM from the 700R4 and very little drivetrain loss with a locking converter and smaller 10 bolt rear will also be huge for mileage. There's no reason a setup like that couldn't be well over 20mpg as long as you keep your foot out of the skinny pedal. There are quite a bit of resources available for performance mods to the 250.

- I would personally go in this order of build if I'm starting at a 250/700R4/14BSF 3.73.
- Make sure TCC lockup is functioning properly. Record starting mileage.
- Change out rear end for 10 Bolt w/3.05 gears. Go drive it and record mileage.
- Fine tune timing and carb. Record MPG during adjustments.
- Install EFI and/or tunable ignition components(holley makes a hyperspark system that would give you full EFI timing tunability).
Alternatively you could do a performance intake manifold/carb setup that can be tuned(quadrajet is probably best for MPG tunability).


Here's a few hypotheticals:

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[Ricko1966]

The factory 3speed OD transmissions were not a hang off the back deal. The overdrive was built into the transmission,was electrically operated and would freewheel instead of engine brake when you let off the gas. I'd think it would get as good or better than a 700 r4. They came with some steep 1st gears and no slippage at all,other than taking off. @Dejure has first hand expierience running one in a truck. I've looked for them would love to have one,it just hasn't happened. Another good manual transmission for your goals would be an np833. 3.09 1st gear and I think a .73 overdrive. GM built a 305 squarebody with an np833 and claimed 28 mpg. I don't think anyone really ever got that though

 

[TotalyHucked]

The factory 3speed OD transmissions were not a hang off the back deal. The overdrive was built into the transmission,was electrically operated and would freewheel instead of engine brake when you let off the gas. I'd think it would get as good or better than a 700 r4. The came with some steep 1st gears and no slippage at all,other than taking off. @Dejure has first hand expierience running one in a truck. I've looked for them would love to have one,it just hasn't happened. Another goid manual transmission for your goals would be an np883. 3.07 1st gear and I think a .73 overdrive. GM built a 305 squarebody with an np883 and claimed 28 mpg. I don't think anyone really ever hot that though

Closest I ever got in my truck as 24mpg. That was a whole tank in semi-heavy traffic running a steady ~65-70 for a few hours straight, normally we run 80+. That was with the 5.3/T56/3.90/20s combo

 

[DoubleDingo]

The factory 3speed OD transmissions were not a hang off the back deal. The overdrive was built into the transmission,was electrically operated and would freewheel instead of engine brake when you let off the gas. I'd think it would get as good or better than a 700 r4. The came with some steep 1st gears and no slippage at all,other than taking off. @Dejure has first hand expierience running one in a truck. I've looked for them would love to have one,it just hasn't happened. Another good manual transmission for your goals would be an np883. 3.07 1st gear and I think a .73 overdrive. GM built a 305 squarebody with an np883 and claimed 28 mpg. I don't think anyone really ever hot that though

The saginaw 3 speed with borg warner overdrive is a good setup, but has 30% reduction, so small tires are needed. Crusty Biscuit, 4.57 gears, 305, gets 12 in overdrive or not. Tires are too big diameter for the 30% reduction in rpms, unless it very flat ground. 4.57 X 0.70 = 3.19 3.19 is not a bad gear, just need the torque to push it, especially with the tires I have on Crusty, which are only 30.5" diameter. Even with 4.57 gears, that 2.84 1st gear requires a lot of clutch slipping to get going, especially on uphill surfaces. I was taking a load of compost to mom and pop's one day, went the back way to avoid the crazy a$$holes on the main road, and almost stalled on this curve. Caught it just in time, but lost all momentum, and could barely get moving forward again. It was only a yard of compost, nothing extremely heavy, but lots of clutch slippage to get going forward again.

The borg warner overdrive is nice, I will admit that, 3000 rpms get dropped down to 2100 in overdrive, quite the drop.

If I were the OP I'd go with 700r4 or the like, but if they can find a borg warner overdrive transmission, with the tire sizes they mentioned, it would be a good fit as long as they have 4.56/4.57 gears.

EDIT: I forgot to insert the image of the hairpin corner that I struggled getting the truck going forward again. It's steep, sharp right turn, with maybe 20 feet vertical gain around the curve. The image is about half-way through the turn.

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[Ricko1966]

The three speed saginaws had 4 different 1st gear possibilities the lowest being 3.11 and 3.50 that would change things a little.