SM465 X 2.5 gears

[boltbrain]

If I replace my 4:10 differential gears with 2.5s, I think I get the same engine-to-wheel ratio as a lot of five speed manuals or 4 speed automatics. Id still have low from the NP205 transfer case.
I’ve got a 1979 3/4 ton with a 14 bolt rear end. I’ve heard of swapping in 3.73s, has anyone seen 2.5s? I can’t remember the exact ratio to be seen but there is one about 2.5 available for the 14 bolt. I asked this question on some other thread but thought I’d put it out there to more readers. Thanks

 

[Shorty81]

My SM465 with a 208 transfer hooked to 3:73 with 31.5 tires gets me 55 mph at 2000 rpm. 65 mph is 2500 rpm. I don't go much over that speed. Engine is very happy with this gearing.

 

[TotalyHucked]

Any gear ratio that starts with a 2 is terrible. My '85 had a 2.73 and it was a dog. What's your overall combo? Engine/trans/tire size?

 

[Strick]

If I replace my 4:10 differential gears with 2.5s, I think I get the same engine-to-wheel ratio as a lot of five speed manuals or 4 speed automatics. Id still have low from the NP205 transfer case.
I’ve got a 1979 3/4 ton with a 14 bolt rear end. I’ve heard of swapping in 3.73s, has anyone seen 2.5s? I can’t remember the exact ratio to be seen but there is one about 2.5 available for the 14 bolt. I asked this question on some other thread but thought I’d put it out there to more readers. Thanks

79 Crew with 350/SM465 & 3.73 with 265/75/16's gets me 12MPG avg. I didn't know a numerically lower gear was offered for a 14FF. I'd stay away from any 2.## gear myself.

Strickland

 

[nvrenuf]

The big question here is WHY?

The highest ratio available for a 14 bolt is a 3.23 and the highest I've seen for a 10 bolt front is a 2.73, I've never seen a ratio comparable to the rear 3.23. 1st gen Dodge trucks with a Cummins could have a 3.07 ratio D61 & D70 combo.

As others have stated, IF you were able to find some sort of 2.xx combo would be a recipe for disaster. The transmission would likely run hot from being under geared not to mention the stress on u-joints, etc..

 

[TotalyHucked]

I should add, a NUMERICALLY lower (which is referred to as a "taller" or "longer" gear) gear ratio brings the RPMs down on the interstate but makes the truck work harder in town/to get up to speed. A 2.5x gearset would make that truck work so hard it will hate life and be a slug anywhere you go. Like @nvrenuf said, the trans will be very hot all the time too.

A NUMERICALLY higher (referred to as a "shorter" or "lower" gear) gear ratio helps in town mileage/power/get up and go but raises the revs on the interstate. It's a balancing act, you want to find a good middle ground.

For example, in a stock small block/overdrive combo, I think a 350/700R4/3.42 is about as perfect as it gets. Not buzzing too bad on the highway but can get out of it's own way. My buddy's 350/700R4/2.73 truck he just bought feels like it has ZERO power until you're rolling 50+.

Or, as another example, my truck is LS powered and LS engine make their power and torque higher in the rev range, so I've got a 5.3/4L65/3.90 combo. It's about perfect, gets good highway mileage and has good in town power.

 

[boltbrain]

I should add, a NUMERICALLY lower (which is referred to as a "taller" or "longer" gear) gear ratio brings the RPMs down on the interstate but makes the truck work harder in town/to get up to speed. A 2.5x gearset would make that truck work so hard it will hate life and be a slug anywhere you go. Like @nvrenuf said, the trans will be very hot all the time too.

A NUMERICALLY higher (referred to as a "shorter" or "lower" gear) gear ratio helps in town mileage/power/get up and go but raises the revs on the interstate. It's a balancing act, you want to find a good middle ground.

For example, in a stock small block/overdrive combo, I think a 350/700R4/3.42 is about as perfect as it gets. Not buzzing too bad on the highway but can get out of it's own way. My buddy's 350/700R4/2.73 truck he just bought feels like it has ZERO power until you're rolling 50+.

Or, as another example, my truck is LS powered and LS engine make their power and torque higher in the rev range, so I've got a 5.3/4L65/3.90 combo. It's about perfect, gets good highway mileage and has good in town power.

Thanks, what year is your truck with the LS? Maybe I should get one of those. Mileage, especially on the highway is what I need. I don’t understand the comments about lugging it from going to taller gears, since with say the 2:73 I’d just be using first for second instead of for granny. And using fourth for fifth, etc. in other words, using the SM like a five speed that lost it’s first gear, which on the NV4500 is 6.34. The SM first is 6.55. Like I said before, going to 2.5 gears just moves all the tranny gears up one gear. So I don’t see any lugging but I am not an expert. I had a 77 half ton with a 292 with 2:73s once and that got decent mileage, maybe 15, but this is my first 350 and I have no idea what the mileage is. The gas gauge doesn’t work and I’ve got a more reliable truck I drive.
I just need a spare hauler and don’t consider 4:10s with a four speed a hauler. Just getting spoiled by five speed manuals I guess. I’ve talked to Advance Adapter about putting a NV4500 in it to get the overdrive (5th) but that means either swapping out my NP205 for a 241 or swapping the NV4500 tail for a AA adapter tail for the 205.
I might just put 2:73 ring and pinion in the rear, take the front driveshaft out, and see how it works.

 

[TotalyHucked]

My truck's an '85, I swapped in a 5.3/4L65 out of an '06 Envoy Denali. It's got a cam, stall converter and then the 3.90 gears. It's lowered on 20s with a 315/35/20 in the back. I get 13-17mpg around town depending on my foot and 18-22mpg on the highway depending on how fast we're running and if we're on flat ground or in the mtns.

I get what you're saying, but that's not exactly how it works. If you go too far with a gear change, it negates what you're trying to do. Yes, if you go to a numerically lower gear, it *can* help mileage. But if you go too far, you'll hurt both your power and mileage. You also have to take into consideration tire size/how much torque the engine makes/how healthy the engine is. If you're running a stock sized tire, I would say put a 3.55 or so in it. That would be a good middle ground. But a 350 doesn't have enough torque for that 2.something gear so it's going to have to work alot harder to move the truck.

My 2wd/single cab/shortbed fairly light truck on stock 235/75/15s got 10-12 with that 2.73 gear no matter what. With your 4wd, there's even more rotating mass and weight to push around, so you'll likely get worse than that. You need the mechanical advantage of a numerically higher gear. Think of it like when you're trying to get a stubborn bolt out that won't budge. A 2.73 gear is like using a stubby 3" ratchet and a 4.10 is like using a 4ft breaker bar. It's a whole lot easier with that longer bar, the less the engine has to labor, the better (to a point).

 

[Curt]

I think it’s a good idea in theory,but the jump from 1st to 2nd on a SM465 is long.It would take a brute of a motor to take up the slack.Towing would be a sh@t show.

The SM465 is a 3 speed + granny.I don’t even use it hunting.So technically I’m saying the jump from 2nd to 3rd

 

[Turbo4whl]

The tallest gear GM used was a 2.56:1 in a 10 bolt C10. It shares the same differential carrier as the 2.73:1 gear set. Two more carriers for the GM 10 bolt, middle gears and the lowest gears. Unless your K20 has a 10 bolt rear axle, I doubt you will find gears this tall. Then you have the issue of the front gear set.

 

[85K304SPD]

Think about it like peddling a 10 speed bike. It takes lots of effort to start off in 10th gear and lots of effort (torque) to maintain speed under load. Your "motor" has to keep struggling the whole time to keep it going. I hope this helps.

 

[Redfish]

I am not sure what exactly you are trying to accomplish with this theoretical gear swap. It doesn't matter what gear ratio you have or how low you drop the RPM, your fuel mileage will not improve to match that of a modern vehicle. You can gear it to turn only 1500 RPM at 55mph but the engine will be using just as much fuel as before but will require more throttle to get up to speed. The squareness of the Square Body doesn't slide through the air very efficiently. An LS Swap is about your best option if you want to get better fuel mileage. A modern fuel injection system would probably help you more than a gear swap.

I cannot imagine any benefit you expect to gain from a gear swap being worthwhile enough to remove your front driveshaft and thereby disable your 4wd capability.

 

[Grit dog]

Trying to get low rpm’s and good mileage and also maintaining some semblance of power and acceleration in a 40-50 year old 4wd brick with engine technology from the 60s is largely an act in futility.
Montana is a big state. Trying to make an old stock squarebody have, really any of the desirable traits of newer pickups brings to mind the old adage.
“Cheap, fast or good. Pick two.”

 

[rpcraft]

With a sm465 about the lowest I would ever venture is 3.55 and that would be no towing or very rare if ever. if I were using it as a work truck I would stick with no less than 3.73. If you really want to cruise and do not need the beef of a nv4500 you can check into a nv3500 but since those are terribly weak units you might also want to investigate a T5 swap.


Also, a bit of a curveball and probably much lesser known on a small block, but you might be able to check into an AR5 out of around a 2007 or 1st generation Colorado. I'm not sure how swap friendly they are on a Small block but they are slightly popular with the LS crowd as an alternative to a t56. They are far cheaper is what it boils down to. The setup I have is for an LS swap and includes an adapter plate between the AR5 and the mount for the hydraulic throwout bearing, on top of a 4l60E bellhousing, then it uses the clutch stuff for the vette LS motors, and a colorado hydraulic setup. I am sure that it is just a matter of measuring and either having the adapters made thicker or having them machined down some if needed, and it might be a case of contacting the guy who makes the adapter and some time to process.

I'm doing one right now going into my LS swapped 64 C10 Pre-runner build so kind of apples to oranges but while it is off I can get thickness dimensions. I just don't have a small block handy to check and test fit is all.

Sorry if I added mind fuel to the fire that will keep you awake at night, but I figured it's worth throwing the idea out there if you want options of gearing and cruising.

 

[SquareRoot]

I say go for it. We can all live vicariously thru the the OP's discovery. An LS for the heck of it and a 14 bolt with 2.50 gears! Subscribed.