TH400 Needs Overdrive... 4L80E swap? Bolt on OD?

I have wasted so much money trying to get good mpg out of big trucks - mainly after believing peoples fairy tale internet mpg claims. Once it gets to real world driving, there just isn't a lot that we can do (that makes sense money wise) to get ahead.

Now... If you want to mod it to make it drive or pull better, and can get a bit better mpg out of it, that is a different ball game, and is fun.

Swap out the gears in your differentials. It'll work a little more off the line (maybe a higher stall TC) but you can get about the same final gear ratio as a 4 speed.

Good idea Peter! I think many people forget about how gearing and the size of the tires effects MPG.

letitsnow said:

I have wasted so much money trying to get good mpg out of big trucks - mainly after believing peoples fairy tale internet mpg claims. Once it gets to real world driving, there just isn't a lot that we can do (that makes sense money wise) to get ahead.

Now... If you want to mod it to make it drive or pull better, and can get a bit better mpg out of it, that is a different ball game, and is fun.

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I think I figured out myself that MPG is a direct function of vehicle weight, at least for a naturally aspirated engine.

It takes a lot more fuel to accelerate and maintain speed of a heavier vehicle than a lighter one... it's that simple. Engine, transmission, and to more or less extent gearing play less role.

I'm actually wondering now what the MPG difference between a TH400 vs 4L80e would be in one of those trucks, but I don't think it would be a huge difference.

Actually, even with all the technology advancement, a NEW Silverado 3500 with a 6.6 gas engine "only" gets around 14 mpg published! Real life is probably worse. I'm pretty sure those trucks are heavier and obviously more capable than yesterday's trucks, but still. Direct injection, advanced timing/ECU parameters, 10 speed transmission, etc.

I still don't understand why OP bought a heavy beast to be daily driving it empty... I'm sure there's a reason.

My burb routinely gets above 17MPG. my cousins 2018 Silverado with the 5.3 was honestly not much better on a 2 hour trip we went on. I got him to switch the truck to MPG and he was getting around 15, so he then started slowing down and setting cruise control which then got him up to 19MPG.

I was laughing my ass off! all that bullshìt AFM OR DFM tech and in the real world it didnt make a lick of a difference because he had a lead foot.

But I dont and I always drive with cruise control. I'm never in a hurry and really dont speed much. impatient people dont like driving with me. haha

Big block engines produce torque at low RPM. You'll need to wind the small block up another 1500+ RPMs to achieve similar torque.
I'll take a big block without OD, at a near constant 2400 RPMs on a trip over a small block
hunting for gear,
shifting from OD,
unlocking the converter,
to 3rd to OD to lockup,
to unlock, to 3rd,
2200 to 4200, to 3000, to 2200 to...
on every hill.

SasquatchOnDeck said:

If it was 4x4, i'd consider swapping it's 350/700R4 into this truck, it would still suit my needs fine, i'm not going to be hauling houses up mountain trails THAT often.

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That would be a terrible swap.... under powered, prone to breakage, and bad on gas.

Doing a manual valve body conversion on a 4L80E is very close in cost to the computer controller.

For my truck I would do the manual conversion. For the Momma's truck I would get the computer.

Her BBC dually with what I think were 4.56:1 gears and 33" tires got 10 MPG consistently. I can't wait to see what a 305 small block gets. Then I'll put the 4L80E in to compare.

454cid said:

That would be a terrible swap.... under powered, prone to breakage, and bad on gas.

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Really? I figured after the dually delete with a wood flatbed, the thing would be light enough that a 454 would just be overkill and a 350 would do fine, but then again, that's why i'm here, i'm just guessing.

The 350 will probably be okay, after all, there were plenty of 3500s that came with 350s...700R4, that's a different story!

BTW, what is your gear ratio on your truck?