4.56 to something

[Adamds86]

Hello everybody, I have a 73 GMC K25/2500 with a SM456 trans. In the rear end it has 4.56 gears. I am wondering how hard it would be to swap these out with something like 3.56 or 3.73? So that it is a little easier to drive on the highway, I don't need the 4.56 as I am not going to pull anything. Also do I need to change the gearing in the front diff as well?

Thanks for the help

 

[skysurfer]

Tire size and desired cruising speed needs to be figured into the equation, and yes, front diff has to match the rear.

 

[Adamds86]

Right now I have 265/75/16 tires. Desired cruising speed is 60 MPH

 

[skysurfer]

Those tires can be figured at 31.5" (close enough) so 3.56 gears would have you running 60.5 mph at 2300 rpm. Seems about right, passing at 70 mph you would still be at a reasonable 2700 rpm.

For comparison, your 4.56 gears currently have you going 55.5 mph at 2700 rpm.

 

[HotRodPC]

Personally, with all the money you'd spend on swapping 2 sets of gears, and IIRC, you'd have to also replace the carrier in the rear since 4.56 up numerically, and 4.10 down is the cut off, I'd say the money is better spent going to an NV4500 5 speed transmission and keep the 4.56's so you have the better of all worlds. That would give you 2100rpm at 60mph where as now you're at 3000rpm at 60. The NV4500 would give you 2400rpm at 70mph. You'd still have the granny gear and the 4.56 for all the low end grunt. 4.56's also get good mpg for city mpg. The added OD will give the mpg on the highway.

 

[Adamds86]

I will ask another question that might solve this too. What about swapping out the SM456 for a TH400 or TH350 automatic? In order to do the swap from manual to auto would I need to change the transfer case? Sorry for all the questions, I am new to old classic trucks lol. Thank You.

 

[DoubleDingo]

TH400 or TH350 will be the same top end rpm's as the SM465 only no granny low. NV4500 is the money best spent.

 

[1low4x4]

if on a budget, pull the front driveshaft out and do your gear swap in the rear, and worry about the front later

 

[Jrgunn5150]

What are you doing with this truck? What are your goals?

 

[Adamds86]

What are you doing with this truck? What are your goals?

I am going to fix it up so that it will be a clean truck. It will not be in the mud much. Just want it to be a Sunday cruiser and a truck that I can go to Lowes or Home depot and pick things up in etc. It probably won't leave the pavement very much. Don't really need it to pull anything.

 

[Jrgunn5150]

I am going to fix it up so that it will be a clean truck. It will not be in the mud much. Just want it to be a Sunday cruiser and a truck that I can go to Lowes or Home depot and pick things up in etc. It probably won't leave the pavement very much. Don't really need it to pull anything.

In that case I don't think I'd throw a ton of money at it.

So first, just accept the slow lane. You can do that for free.

Second, bigger tires. They look sweet, and lower your effective ratio. 35's and 4.56 are a decent combo.

Third, you could find someone local with a pair of 3.73 geared axles that will swap with you. I know I would in a heartbeat, most offroader's would.

An overdrive trans swap will get over 1,500 bucks real quick, no matter which route you take in doing it. Regearing your current axles wouldn't be alot more affordable either.

 

[HotRodPC]

I will ask another question that might solve this too. What about swapping out the SM456 for a TH400 or TH350 automatic? In order to do the swap from manual to auto would I need to change the transfer case? Sorry for all the questions, I am new to old classic trucks lol. Thank You.

TH400 or TH350 will be the same top end rpm's as the SM465 only no granny low. NV4500 is the money best spent.

Yep, actually even worse cuz you lose 300rpm or so in slipping converter. You don't get a full 1 to 1 ratio like you do in a manual unless you have a lock up conveter. So one of those is even worse yet. If you want to go Auto, 700r4 will be an OD with lock up converter, but it's not as durable as the NV4500.

 

[4WDKC]

If we were closer id swap you for my 1/2 tons axles w/342s and some cash for your 456 3/4 ton axles.