Changing gear ratios: opinions/suggestions wanted!!

My 78 K20 4x4 has factory 4:56 gears in it and a TH400 transmission. These are great when off-road or pulling something, but poor on the highway. I grew up with a 75 half ton 4x4 with 3:73 gears and a 4 speed manual, which was very versatile for both off-road and highway, but that was partly due to the 4 speed manual transmission. I'd like to switch out my low gears for something more versatile, but I can't decide on 3:73s or 4:10s, as I don't have an experience with either ratio in combination with the TH400, since I just bought the truck a few weeks ago. I'm leaning toward 4:10s, as I used to have a 74 F150 with something close to that combo, and I liked it. But, the 3:73 is the more common one and gets better mileage, etc. We've touched on this in another thread, but I'd really like some more thoughts from other folks who have experience with either. Thanks!

A 4l80e with a standalone computer would be pretty slick here, otherwise I would go 4.10s.

I would put some 35" tires on with the 4.56 that you have.

So tell me more about the tranny swap options - 4L80E or 700R4? Is that less work/cost than swapping gears and carriers?

I want to go 4L80e with a standalone.

Anyone use these guys before?

Gear swap would be 3.73. My 81 has 4.10s, and they are fine, but only around town or short runs on the highway where I can keep it around 60-65 tops. Highway, the engine is turning pretty good, more than I'd like while going so slow. 3.73s will drop the rpms down some. You have enough slippage with the TH400, 3.73s would be ideal for what you're doing with it. Yes, 4L80E and no gear change gives you the same gearing down low and overdrive for the highway. It all depends what you want to spend your money on and what you want to modify to get the results you're looking for.

DoubleDingo said:

Gear swap would be 3.73. My 81 has 4.10s, and they are fine, but only around town or short runs on the highway where I can keep it around 60-65 tops. Highway, the engine is turning pretty good, more than I'd like while going so slow. 3.73s will drop the rpms down some. You have enough slippage with the TH400, 3.73s would be ideal for what you're doing with it. Yes, 4L80E and no gear change gives you the same gearing down low and overdrive for the highway. It all depends what you want to spend your money on and what you want to modify to get the results you're looking for.

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Thanks all for the comments, much appreciated!

@DoubleDingo - The overdrive in the 4L80E would be nice, but the folks at Auburn gears told me that a 700R4 would actually bring the RPMs down even further (about 400 RPMs lower than the 4L80 @ 60 MPH) and would be an easier/more direct swap (so probably cheaper), though no O/D, of course. See pics.

As for gears, I priced both ring & pinion sets at TorqueKing, and the 3:73s would be cheaper to buy, but they thought those would require new carriers, too, maybe making it no savings over the 4:10s. The 4:10s would not need new carriers, according to them, so maybe a wash.

I'm starting to think since both Auburn and TorqueKing suggested swapping trannies, that maybe I should be looking into that harder.

BigT said:

Thanks all for the comments, much appreciated!

@DoubleDingo - The overdrive in the 4L80E would be nice, but the folks at Auburn gears told me that a 700R4 would actually bring the RPMs down even further (about 400 RPMs lower than the 4L80 @ 60 MPH) and would be an easier/more direct swap (so probably cheaper), though no O/D, of course. See pics.

As for gears, I priced both ring & pinion sets at TorqueKing, and the 3:73s would be cheaper to buy, but they thought those would require new carriers, too, maybe making it no savings over the 4:10s. The 4:10s would not need new carriers, according to them, so maybe a wash.

I'm starting to think since both Auburn and TorqueKing suggested swapping trannies, that maybe I should be looking into that harder.

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700r4 is overdrive,steep first gear,not as durable as 4l80. But either way a transmission swap makes more sense than 2 gear sets and diffs. 700r4 will be a cheaper swap anyone going stand alone on 4l80e make sure your controller controls line pressure also,or use transgo 48mod to control line pressure.

Ricko1966 said:

700r4 is overdrive,steep first gear,not as durable as 4l80. But either way a transmission swap makes more sense than 2 gear sets and diffs. 700r4 will be a cheaper swap anyone going stand alone on 4l80e make sure your controller controls line presdure also,or use transgo 48mod to control line pressure.

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Thanks! I didn't know 700R4 had O/D too. Never had either 4L80E or 700 before. Every SBC I've ever driven until now had a manual behind it...

BigT said:

Thanks all for the comments, much appreciated!

@DoubleDingo - The overdrive in the 4L80E would be nice, but the folks at Auburn gears told me that a 700R4 would actually bring the RPMs down even further (about 400 RPMs lower than the 4L80 @ 60 MPH) and would be an easier/more direct swap (so probably cheaper), though no O/D, of course. See pics.

As for gears, I priced both ring & pinion sets at TorqueKing, and the 3:73s would be cheaper to buy, but they thought those would require new carriers, too, maybe making it no savings over the 4:10s. The 4:10s would not need new carriers, according to them, so maybe a wash.

I'm starting to think since both Auburn and TorqueKing suggested swapping trannies, that maybe I should be looking into that harder.

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Overdrive ratio between the 700r and 4L80 is .70 vs .75. Barely noticeable. Those RPM numbers are off.

PartsMan79 said:

I would put some 35" tires on with the 4.56 that you have.

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That’ll still be basically useless on the highway. 4.10s and 35s are 3krpms at 70mph 1:1.

BigT said:

Thanks! I didn't know 700R4 had O/D too. Never had either 4L80E or 700 before. Every SBC I've ever driven until now had a manual behind it...

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Top gear is still 1:1 unless you have a rare A833 but those were only in a few 1/2 tons.
Trans will cost more than gears. And will be about the same result. But more mods than just gears. No “easy” answer here, really.
I’d actually be more inclined to swap a 5 or 6 speed manual. Again being 4wd complicates that as well.
Unless you’re looking to really try to economize your highway driving (and still get pretty schitty mileage) gears are the simplest, basically bolt n go.

Grit dog said:

That’ll still be basically useless on the highway. 4.10s and 35s are 3krpms at 70mph 1:1.

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Despite popular belief turning 3k on the highway is not a problem. Hell I've turned 3800-4000 for hours on end...

77Dmax said:

Despite popular belief turning 3k on the highway is not a problem. Hell I've turned 3800-4000 for hours on end...

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Oh I agree, it’s not gonna blow up the engine in the near future.
But that’s not the point I was trying to make nor likely the reason the OP wants lower highway rpm’s.
If it was ideal or even fun to have the engine screaming down the highway all the time, I’m betting 4,5,6,7,8,9 and 10 speed light duty transmissions wouldn’t have been popular enough to advance and be the norm from the 1980s to 2020s.

The OD ratios in the newer 6, 8 and 10 speeds are not much higher than what was offered in the 80s.

I'm not here to argue but turning 3k at 70 is far from useless and is acceptable.