86 1/2 ton gears needed for 33” with new 383

[Vbb199]

4.10

 

[PrairieDrifter]

I have 350tbi/4L60/3.73 in my burb with "33's" that actually tape out around 32 inches. it is adequate enough that I won't likely regear but will look to add about 50 more hp when the engine dies. really it isn't bad on the highway cruising around 2100 RPM at 74-75mph flat ground. it does unlock the converter on mild hills and downshift on more significant hills, but it doesnt seem that much different than my 2004 burb, honestly. what really hurts drivability is any significant cross/head wind. that really kills it on the highway and I usually back off the speed a touch to help. I do about 15k miles a year in mine, with over half that spent on the highway between home and the farms (300mile one way).

if I was in a position with 2.73 gears and was going to regear for sure, I'd be a tough call between 4.10 and 4.56, with a slight nod toward 4.56 probably. that'd put me around 2400rpm cruising at 74-75.

I'd play with the rpm calculator with actual tire sizes and see what you think you want based on the engine's power band.

Gear Ratio Calculator

Definitely on par. A nice clear day with no wind and the truck will do whatever you want pretty much without unlocking the converter.


Wind affects these trucks pretty bad, and at 60+ mph, then they turn into a dog, especially with hills.

If anything it's liveable. Especially if you're not doing long interstate road trips. Highway is usually fine, the hills can get annoying but, $2000 minimum to regear isn't a small thing for most people.

 

[bucket]

Definitely on par. A nice clear day with no wind and the truck will do whatever you want pretty much without unlocking the converter.


Wind affects these trucks pretty bad, and at 60+ mph, then they turn into a dog, especially with hills.

If anything it's liveable. Especially if you're not doing long interstate road trips. Highway is usually fine, the hills can get annoying but, $2000 minimum to regear isn't a small thing for most people.

Yep, with no headwind and a flat interstate, it's fine. I was never so lucky though. It seemed like I always had a headwind and was going up some kind of incline on any interstate trip. Just like grandpappy had to walk to school, in the snow every day of the year and it was uphill both ways, lol.

 

[PrairieDrifter]

Yep, with no headwind and a flat interstate, it's fine. I was never so lucky though. It seemed like I always had a headwind and was going up some kind of incline on any interstate trip. Just like grandpappy had to walk to school, in the snow every day of the year and it was uphill both ways, lol.

That's my problem as well didn't matter if I was going back the way I came a few hours earlier, seems like the wind is always blowing at me lol.

I liked the 04 burb comparison above. Super relatable. Our 08 Tahoe was definitely more powerful, but smaller, newer and lower mileage.

Also it's not apples to apples, but my old daily (06 Colorado with 5 cylinder) stock 31's was SUPER comparable to my burb on 35's. That's why I've been completely fine with the 3.73's 700 combo. The 06 was better on interstate holding speed but not much. Just about as bad in the wind. But of course the burb feels better down low and in town. PLUS I'm getting the same mileage if not a little better than the mini truck... that was over 260,000 miles too.

I assume with a fresh mild 350 and 4.10's, I would be completely satisfied for my usage. For a suburban that wouldn't do any real towing. A truck that I would daily plus tow with every now and then, I would go 4.56's.

Is there an in between from 4.10-4.56? That seems like that would be the ticket.

 

[gmbellew]

That's my problem as well didn't matter if I was going back the way I came a few hours earlier, seems like the wind is always blowing at me lol.

I liked the 04 burb comparison above. Super relatable. Our 08 Tahoe was definitely more powerful, but smaller, newer and lower mileage.

Also it's not apples to apples, but my old daily (06 Colorado with 5 cylinder) stock 31's was SUPER comparable to my burb on 35's. That's why I've been completely fine with the 3.73's 700 combo. The 06 was better on interstate holding speed but not much. Just about as bad in the wind. But of course the burb feels better down low and in town. PLUS I'm getting the same mileage if not a little better than the mini truck... that was over 260,000 miles too.

I assume with a fresh mild 350 and 4.10's, I would be completely satisfied for my usage. For a suburban that wouldn't do any real towing. A truck that I would daily plus tow with every now and then, I would go 4.56's.

Is there an in between from 4.10-4.56? That seems like that would be the ticket.

there is a rare 4.30ish something, but it isn't commonly available. with 32-33in tires, that would just about put it like stock tires and 3.73s.

I bet 35s and 4.56 is probably close to a "stock" gearing/tire size for 3.73.

 

[PrairieDrifter]

there is a rare 4.30ish something, but it isn't commonly available. with 32-33in tires, that would just about put it like stock tires and 3.73s.

I bet 35s and 4.56 is probably close to a "stock" gearing/tire size for 3.73.

I figured as much on the in between.

The math seems like it would pretty much add up to stock without doing any of it lol. A lot of it comes down to preference and usage really.

 

[MikeB]

Have you confirmed the 2.73 gears? Seems strange to me for a K10 with a 700R4 (if that's what it came with), but GM was very worried about gas mileage back then.

As for gear ratio selection, check out some on-line calculators to determine RPMs at various road speeds to see what you can live with. And having run a mild-performance 383, I'd say you can get by with 10%-15% taller gearing than with a 350. Even more compared to a 305.

 

[Vbb199]

Someone mustve swapped the 700 in, a 2.73 wouldve been paired with a th350

 

[Grit dog]

Someone mustve swapped the 700 in, a 2.73 wouldve been paired with a th350

Not according to the GM Heritage vehicle info kits.
5.7 auto truck was OD trans only. And only 2.73 or 3.08 gears.
Also curiously the only 5.7 power team was with CA emissions. Didn’t shown any 5.7 federal emissions trucks.
Now the 4.3, 5.0 and 6.2 had far more options for gearing and some with a non OD auto (th350).

 

[MikeB]

5.7 auto truck was OD trans only. And only 2.73 or 3.08 gears.

Man, I can't imagine how sluggish the engine would be in .70 OD with a 2.73. And with the converter locked up -- forget about it!

I would have guessed trucks with 700R4/4L60 transmissions would have had 3.42 as standard.

Edit: I just now remembered renting a Chrysler product from Hertz (or whoever) back in the late 70s/early 80s. When that thing shifted into top gear, the entire car would shudder and the engine would clatter. Absolutely horrible! I still remember driving around everywhere except the freeways in 2nd gear.

 

[Grit dog]

Man, I can't imagine how sluggish the engine would be in .70 OD with a 2.73. And with the converter locked up -- forget about it!

I would have guessed trucks with 700R4/4L60 transmissions would have had 3.42 as standard.

Edit: I just now remembered renting a Chrysler product from Hertz (or whoever) back in the late 70s/early 80s. When that thing shifted into top gear, the entire car would shudder and the engine would clatter. Absolutely horrible! I still remember driving around everywhere except the freeways in 2nd gear.

Ya doesn’t make sense and I could be wrong but lotsa crazy stuff in the 70s/80s in the name of emissions and fuel economy.
Thats just what I gleaned from the vehicle specs archives.

 

[legopnuematic]

lotsa crazy stuff in the 70s/80s in the name of emissions and fuel economy.

Like the 2.14 ratio available in the 7.5” ten bolt axles.

Pinion is almost as big as the ring gear, but I think the land speed guys like them.

You must be registered for see images attach

To the OP, with a healthy 383 that is built up some, 3.73 gears and 33” tires should be ok, but if you are in hilly terrain or will be loaded 4.10s might be a better choice.

Also remember that the cam and torque converter along with gears/tire size all contribute to the “driving” feel as does vehicle weight.

My friends 75 K5 is running the 10 bolts from his 87 and had it regeared to 4.56 when he went to 33” tires, that was with a stock 305/700r4, but it worked ok with that combination.

In the 75 it’s a nice 385, performabilt 700r4, same 4.56 and 33” tires and it really was not pleasant to drive. In my opinion because a full top K5 is fairly light and other factors it was never really loading the converter up and making the best use of it. Now something much heavier like a one ton with the same setup it probably would work out nicely.

Anyways, built a 6 lug 14 bolt semi float for it, went with 3.73, friend is much happier with that than the 4.56 in the 10 bolt. Dropped about 10hp to the tires. Hard to say how much of that drop was the rear end (being larger, more parasitic losses) or the change in ratio.

Just some things to consider to make everything work happily as a combination.

 

[MikeB]

Like the 2.14 ratio available in the 7.5” ten bolt axles.

Pinion is almost as big as the ring gear, but I think the land speed guys like them.

Back in the day I had an 81 C10 with a 305 and 2.56 gears, and thought that was tall!

But many years later I gave a 2.73 gearset from a 7.5" 10-bolt to a guy who had a Pontiac with 2.21 (or 2.29?) gears. He said the 2.73s would be a high performance upgrade!

*** Hey Grit dog -- I wasn't disputing what you said. I just thought the 2.73s would have been terrible with an OD trans in a truck.

 

[JD Miller]

2:73 gears ??.... Never heard of a cheby truck with those

I thought only Ford did dumb stuff like that