hypothetical, mpg

[Matt69olds]

Personally, I would tune what you have. Dial in the cruise AFR, play anround with ignition timing, this is where an AFR and vacuum gauge would be helpful.

Any modifications you make are going to cost many times the amount of money you save in fuel.

A 700 transmission would definitely help both economy and performance. I’m guessing the first gear is considerably lower than the 3 speed, and the overdrive and lockup converter will no doubt help.

 

[DoubleDingo]

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

True. I only have the 4.57 trucks, so they came with 2.84 1st gear, and have only seen zero groove saginaw 3 speeds. Finding the 3.11 or 3.50 first gear versions would be a game changer. Probably easier to find a 700r4.

 

[Ricko1966]

True. I only have the 4.57 trucks, so they came with 2.84 1st gear, and have only seen zero groove saginaw 3 speeds. Finding the 3.11 or 3.50 first gear versions would be a game changer. Probably easier to find a 700r4.

I'm laughing seriously,because of all the years I've had my eyes open for an overdrive saginaw,I've found exactly zero, of any gear ratio. But I can throw a rock in any salvage yard and hit a 700r4

 

[DoubleDingo]

Personally, I would tune what you have. Dial in the cruise AFR, play anround with ignition timing, this is where an AFR and vacuum gauge would be helpful.

Any modifications you make are going to cost many times the amount of money you save in fuel.

A 700 transmission would definitely help both economy and performance. I’m guessing the first gear is considerably lower than the 3 speed, and the overdrive and lockup converter will no doubt help.

Yep, maximum vacuum at cruise speeds. That is why Crusty only gets 12 in or out of overdrive. Out of overdrive, 3,000 rpms and around 55-60 mph vacuum is around 15, same as idle. In overdrive, rpms drop 30%, but the vacuum is around 5 and 10, so more throttle is needed to go the same speed or a shade faster. Granted there is little flat ground here, but still, the vacuum gauge doesn't lie.

 

[DoubleDingo]

I'm laughing seriously,because of all the years I've had my eyes open for an overdrive saginaw,I've found exactly zero, of any gear ratio. But I can throw a rock in any salvage yard and hit a 700r4

I only have the saginaw 3 speed with borg warner overdrive because it came installed on Crusty when I bought it. But, the other '65 3/4 ton I have came with a regular saginaw 3 speed. I have used both units in Crusty, and verified mileage with both on long trips that went to the same areas and back.

 

[Dejure]

Around the time I ran that overdrive out of an old 62 wagon in my 69 C10, I was all over the board on what I tried.

A kid opened a junk yard and focused on the likes of all of us. My first experiment was, I installed a 4spd out of a GOAT. The gears were WAY to tall for that little six, even with the 4:13 rear end, so I took it back and tried a Lemans 4spd. It was perfect.

Still wanted better. That's when I learned about the 3spds with overdrive. Got it installed with a Hurst floor shifter. Wrapped the handle in wood, with a flair at the top, which held the rocker switch that controlled the overdrive.

The 3spd overdrive made for a sweet ride. Shifts were easy, whether from the mechanical or electrical approach.

As Rick noted, they liked to freewheel. Learned that the hard way. Parked on the ferry between Bremerton and Seattle and went up for coffee. Came back to find my truck bumping between the rigs surrounding me. The rig in front of me was a big truck and the driver was in it. Wondered why he was grinning at me, as I got to my truck. Didn't take long to figure out why. Just lucky it wasn't a serious stormy day. No damage done.

All I did to the engine was, added a mild RV cam, installed headers and swapped the straight, 7amp points for capacitive discharge (dropped the point amperage to 200 milliamps and the points lasted so long the rider lobe wore off). Still managed 25 mpg one day, but I was playing the proverbial granny to get it (probably had a tailwind too). With today's tech (air dams, injection, radials, slicker paint (okay, maybe not that one) and some mods, I believe I could have pushed the 30 mark without having to go all in on what I'm trying to stay away from today (insane electronics).

Since my wife refuses to go the stick route, I'd look down the T-10 Hone overdrive versions today. It'd mean knocking a bit off the drive line, but that would give and excuse to balance it at the same time. And wifey would likely only run it off the automatic, so my mileage would be better than hers.

 

[DoubleDingo]

Around the time I ran that overdrive out of an old 62 wagon in my 69 C10, I was all over the board on what I tried.

A kid opened a junk yard and focused on the likes of all of us. My first experiment was, I installed a 4spd out of a GOAT. The gears were WAY to tall for that little six, even with the 4:13 rear end, so I took it back and tried a Lemans 4spd. It was perfect.

Still wanted better. That's when I learned about the 3spds with overdrive. Got it installed with a Hurst floor shifter. Wrapped the handle in wood, with a flair at the top, which held the rocker switch that controlled the overdrive.

The 3spd overdrive made for a sweet ride. Shifts were easy, whether from the mechanical or electrical approach.

As Rick noted, they liked to freewheel. Learned that the hard way. Parked on the ferry between Bremerton and Seattle and went up for coffee. Came back to find my truck bumping between the rigs surrounding me. The rig in front of me was a big truck and the driver was in it. Wondered why he was grinning at me, as I got to my truck. Didn't take long to figure out why. Just lucky it wasn't a serious stormy day. No damage done.

All I did to the engine was, added a mild RV cam, installed headers and swapped the straight, 7amp points for capacitive discharge (dropped the point amperage to 200 milliamps and the points lasted so long the rider lobe wore off). Still managed 25 mpg one day, but I was playing the proverbial granny to get it (probably had a tailwind too). With today's tech (air dams, injection, radials, slicker paint (okay, maybe not that one) and some mods, I believe I could have pushed the 30 mark without having to go all in on what I'm trying to stay away from today (insane electronics).

Since my wife refuses to go the stick route, I'd look down the T-10 Hone overdrive versions today. It'd mean knocking a bit off the drive line, but that would give and excuse to balance it at the same time. And wifey would likely only run it off the automatic, so my mileage would be better than hers.

Yeah, that freewheeling is not cool when you're expecting a manual transmission to have engine braking. The PO didn't give me the rundown on how it worked, and he somehow had it to where it would go in reverse. Once I figured out the system and adjusted the hokey cable setup he had, if the cable was in the overdrive position, it no longer went into reverse. Until I figured it out, it went into reverse and freewheeled, which, with the crappy brakes, was scary as heck.

 

[Dejure]

So, DoubleDingo, what you're saying is, in spite of my total ignorance, I got lucky just freewheeling in forward gears, and still being able to back up.

 

[DoubleDingo]

So, DoubleDingo, what you're saying is, in spite of my total ignorance, I got lucky just freewheeling in forward gears, and still being able to back up.

If it's freewheeling, it's not supposed to go backwards. Transmission failure happens. The no reverse is designed into it. Why? The engineers know more than I us non-engineers. The fact that we were able to reverse the respective vehicles means something was amiss.

 

[carnutjw]

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.

First post/question coming in hot! Besides the aero complications, which are kinda "it is what it is", efficiency is the goal. Making the drivetrain as lightweight as possible yet able to handle the vehicle weight/engine torque, and low cruising RPM would be your best bet, that is apparently the late model OEM goal.
The 14 bolt is waaayyy overkill. 10 bolt 3.08 w/OD would be much better. In my opinion the 250, while super durable, is not the most efficient option. I think a 4.3L V6 or small c.i. V8 would do better. Lightweight wheels and tires with low rolling resistance tires (not A/T's or M/T's) can make difference too. Good tuning is critical.
On the aero front- lowered, front air dam, maybe lip spoiler on the tailgate and bed cover, anything to reduce drag at highway speeds.

 

[Dejure]

If memory serves, in OD, it freewheeled. Out it didn't. Don't know that I knew OD would lock you out of reverse, since I never gave a thought to backing up fast.

 

[DoubleDingo]

If memory serves, in OD, it freewheeled. Out it didn't. Don't know that I knew OD would lock you out of reverse, since I never gave a thought to backing up fast.

Correct, it freewheeled when the solenoid wasn't engaged. The reverse lockout is from the rod that is pushed forward from the tailshaft/overdrive side, thus locking out reverse, see attached...

It has nothing to do with shifting quickly or slowly into reverse, it locks it out, and the sun gears in the tailshaft grab the planet, and keep the vehicle from backwards.

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[DoubleDingo]

Reverse locked out

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Reverse not locked out

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[DoubleDingo]

Correct, it freewheeled when the solenoid wasn't engaged. The reverse lockout is from the rod that is pushed forward from the tailshaft/overdrive side, thus locking out reverse, see attached...

It has nothing to do with shifting quickly or slowly into reverse, it locks it out, and the sun gears in the tailshaft grab the planet, and keep the vehicle from backwards.

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I misspoke. It's not the sun and planets gears keeping it from rolling backwards, it the big needle bearings that get pushed out via several cams that push them against the housing.

The big needle bearings are on the blue towel on the bench, and beyond the sun gear of the tailshaft, you can see the smooth section, those big needles get pushed against that smooth section. The big needle bearings sit in the slots behind the planets. Those needles and the cams and the tailshaft section require a lot of patience to get it all back together. People have used rubber bands to hold the needles in the slots, and then had a useless transmission. The trick is grease as glue to keep them in place, and then engage the gears so the input shaft can be rotated opposite the cams (freewheel) and the needles suck in and the tailshaft slides on.

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[YakkoWarner]

If it's freewheeling, it's not supposed to go backwards. Transmission failure happens. The no reverse is designed into it. Why? The engineers know more than I us non-engineers. The fact that we were able to reverse the respective vehicles means something was amiss.

The British overdrive units (which are a bit of a Rube Goldberg setup using a combo solenoid/hydraulic pump engagement system) have a very stern warning about not attempting to reverse the vehicle if the overdrive is not disengaged. Has something to do with the wet clutches being designed to only grip one way from what I can glean. I don't know all the details and I can't quite work out how the thing works to begin with (sun gears, planet gears, gears with teeth on the inside) but trying to run it backwards is apparently a Very_Bad_Thing and those units aren't cheap.