M1028 gear ratio swap input

[moehos]

Hi all,

I am looking for input from folks who have swapped their axle gear ratios from the stock 4.56 to something less aggressive like a 3.73.

I mostly run on the roads, I don't offroad recreationally, but occasionally find myself off the beaten path for work or play, but never crawling. It can snow quite a lot here sometimes and I have been up to the bumpers in snow before and swam along quite nicely with chains with my setup as is.

I run stock tire size and have no plans to put on big tires.

I do pull a M101 type trailer occasionally, with the combination loaded reasonably with sawdust or logs.

My complaint is the engine is really screaming at 55mph and to get anywhere here, you usually have to take the highway and the speed limit on any of our major arteries is 65mph. I get run over driving around at 50mph.

I have a pipeline dream of putting a 3 or 4 speed manual in the truck, but thats waaaay off in the weeds when I have time and money. I think that would pair well with a gear ratio change.

But until then, am I going to really miss the low end grunt with 3.73 or will the bump in mileage and easier cruising make up for the need to plan shifts a little more carefully?

Thanks in advance

 

[Dock Rocker]

You will get your highway speeds but it will take a little longer to get there. Depending on how much you shop around you are looking at $1,200 to $1,500 for the swap front and rear.

My opinion is save a little longer and swap the th400 for a 700r4. It will make the highway much better plus a better low gear for getting going.


Sent from my iPhone using Tapatalk

 

[Recovry4x4]

I agree with Dock. My SECM has the SM465 manual trans and 4.10 gears. Can't climb any of these hills in N GA. Wished I had kept the 4.56 stuff. A 700R4 swap wouldn't be much more money than a gear swap but have tons of benefits. The 3.06 low gear really helps the 6.2. I think it's the best match for a CUCV.

 

[moehos]

Recovry4x4, I was actually reading up on another post where you mentioned you were installing the sm465, did you ever detail that build? I would like to hear more!

 

[Keith_J]

With 33 inch tires (255/85 16), 55 MPH is 2600 RPM. Not exactly screaming, the engine is designed to get maximum power at 3600 RPM. Its only screaming because the intake has no restriction and no silencing baffles.

An overdrive transmission with lockup torque converter is your best bet for mileage. These are high drag vehicles and the slip in the TC means mechanical energy is converted to heat. I would be happy with a lockup torque converter in a TH400 but unicorns don't exist. So it will be a 4L80 for me. I still don't trust a 700R4.

 

[Recovry4x4]

Recovry4x4, I was actually reading up on another post where you mentioned you were installing the sm465, did you ever detail that build? I would like to hear more!

I have not....yet. I'm now facing replacing the grenaded transfer case. I have an NV4500 so I'm considering doing that too. When it's all done and I'm tired of it, I'll do a 700R4 swap. I didn't take pics when I did the SM465 swap but maybe I can with the 4500…

 

[m38inmaine]

I have done both swaps, the 700R4 was much better in my opinion.

 

[moehos]

Keith_J; I have only recently seen the folks on here that did the swap that are stoked about the 700R4, never heard a good thing about them till then... The 4L80 would be very cool, but I'd rather have a manual if I'm going to be doing any substantial amount of re-engineering already.

Stock tires size is actually 235/85 R16, with a nominal total diameter of 31.5 to 32... not sure what that translates to in RPMs but 55mph is certainly higher than desirable for a cruising speed

 

[pmramsey]

Gear Vendors has a great overdrive kit. Not Cheap at $2,600 but bullitproof. the 3600rpm becomes 2600. The 700r4 has its place but issues must be addressed. One might want to add aftermarket cooling. Also, timing is critical. For my money and needs, I went with the Gear Venders overdrive kit on my M1028A2.

 

[Sharecropper]

In my opinion, a correctly built 700R4 is the wiser choice over a Gear Vendors unit (hereinafter GVU) for the following reasons -

1.- GVU provides higher final ratio to rear axle only; 700R4 provides higher final ratio to both axles simultaneously
2.- An inadvertent shifting into 4WD while the GVU is engaged will cause drivetrain lockup and damage; no possibility of damage with 700R4
3.- GVU requires shortening of rear driveshaft with increased pinion angle; 700R4 does neither
4.- GVU is an add-on to TH400 which has a first gear ratio of 2.48; 700R4 has lower first gear of 3.06

My 700R4 was built by Bowtie Overdrive and is rated at 450HP and 450FPT. I sincerely believe it will handle the 250hp my P400 will throw at it.

To each his own. Just my 2 cents.

 

[LT67]

With 33 inch tires (255/85 16), 55 MPH is 2600 RPM. Not exactly screaming, the engine is designed to get maximum power at 3600 RPM. Its only screaming because the intake has no restriction and no silencing baffles.

An overdrive transmission with lockup torque converter is your best bet for mileage. These are high drag vehicles and the slip in the TC means mechanical energy is converted to heat. I would be happy with a lockup torque converter in a TH400 but unicorns don't exist. So it will be a 4L80 for me. I still don't trust a 700R4.

I'm running BFG KM2 255 85 16's on my 86 M1008 and it rolls 55-57mph easy...

 

[Keith_J]

A GV overdrive doesn't provide a lockup torque converter. That's a big loss of efficiency.

 

[LT67]

I agree with Dock. My SECM has the SM465 manual trans and 4.10 gears. Can't climb any of these hills in N GA. Wished I had kept the 4.56 stuff. A 700R4 swap wouldn't be much more money than a gear swap but have tons of benefits. The 3.06 low gear really helps the 6.2. I think it's the best match for a CUCV.

Just curious, where in N Ga can't you climb hills?

 

[Recovry4x4]

I'm in Blairsville and actually, I can get up them but never in 4th and mostly have to use 2nd. Love the 6.2 on flat Earth but here in the mountains, not so much.

 

[LT67]

I'm in Blairsville and actually, I can get up them but never in 4th and mostly have to use 2nd. Love the 6.2 on flat Earth but here in the mountains, not so much.

The route up Blood Mtn and Wolfe Pen gap can get a lil steep.... I wouldn't try 4th on my 79 K20 with a 406 and 4:10 gears. Too much strain on the engine.

Yeah I can see having to use 2nd gear on some of those hills with a 6.2..... and I'd like to have a SM465 in one of my M1008's

 

[Recovry4x4]

I'm a big fan of the SM465 but not so much behind a 6.2. This truck does have 4.10 gears which hurt it a little. Now that the transfer case has grenaded, I may go ahead and install the NV4500 for a while.

 

[moehos]

Actually, there is a 4x4 compatible Gear Vendor unit. What I've read from other users, and seems to make sense is that the use of an aftermarket overdrive on a TH400 will cause even more slippage with the torque converter. So, beyond that issue, there is the astronomical price for an aftermarket part with no reasonable expectation of a long term supply chain and then the additional cost of adapting the driveline.

Either one isnt an attractive option for me personally, both would require a great deal of reengineering of the existing system, if I was to go to that trouble I'd prefer a well set up 4L80, but this has its own particular complications. Even better would be an muncie 3 of 4 speed, I prefer the simplicity, control and durability of a manual transmission.

I would love to hear from anyone who has done a swap with any transmission what was involved and what the complications were. Have any of you guys actually done a write up here?

 

[Recovry4x4]

No write-up but I did the SM465 swap a few years back. I didn't find it worth the effort but my 16 yr old loves driving it. Next is the NV4500. The swap isn't difficult at all.

 

[moehos]

If you do end up doing that, please please please do at least a minimal write up about it

 

[12Bore]

Hi, I am new to the SS Site. I have installed the Axles from a CUCV in a Full size Blazer, so I am familiar with them.

My Question is will a CUCV with 4.10 Gears pull honest 38" Tires decently or should I hold out for one with the regular 4:56s?
Tires will be Michelin 325/85R16 XMLs . it was a Brush Truck for the Air Force and is a crewcab fire Truck, put in Service in 1988. It is the Square body style not the updated Civvy one

I appreciate any Guidance You may have.

12Bore