gear ratio swap options

[75k20]

i found out i have 4.56 gears in my 75 k20. i have a 350/465/205 setup i believe. I was told the truck was a dually from factory but now it is a single rear wheel.

What are my options for swapping to a more highway friendly gear ratio? Maybe 3.73? 4.10? I want to cruise at or around 2000rpm going 55-65mph

Any good budget friendly companies that a college student could do? I’m fairly mechanically inclined so i believe I could do it myself.

 

[ali_c20]

Here are rpm calculators which gives you ball park numbers for the rpm.

https://spicerparts.com/calculators/engine-rpm-calculator

https://purperformance.com/p-29669-rpm-calculator.html

There are lots of videos on youtube for ring and pinion swaps, First check which axles are on your truck.
Ring and pinion swap is not complicated but it takes some time to adjust everything to specs and some special tools.

 

[75gmck25]

In ‘75 the 4.10 gears were very common in a K20, but I believe 3.73 was available as a factory option. For most axles there is a break point (different carrier) for the carrier size and gears. Based on what I could find online, if it’s a 14 bolt FF you might need a different carrier for anything lower (numerically) than 4.56.

 

[wlwarnke]

I know for 12 bolts, they make R&P sets that will allow you to put any ratio on either of the carriers.

Basically 2 options for the ring gear. Thick for 1 carrier and Thin for the other. Same price too IIRC.

I can only assume they do that for 14b also?

 

[nvrenuf]

@75k20 you need to identify your rear axle. Post pics if you need help.

A dually pick up would have a very wide D70HD. A cab & chassis would most likely have a narrow 14 bolt but that would have to be swapped in to your truck as I'm 99% sure there were no 3/4 cab & chassis's.

If it's a 14 bolt (most common 8 lug rear axle and would have come in a '75 3/4 ton) the ration break for carriers is p to 4.10 and 4.56+ so if you want to drop to a 3.73 or 4.10 you'll need a carrier.

 

[Bextreme04]

Even with a 3.73, you will still be at 2400 rpm at 65mph. Having the 465 gives you a crazy low 1st and 2nd gear. I would take advantage of that to put something really tall in it for highway driving. You can get a 3.42 for a 14BFF for $225 on rockauto. Probably another $150-200 for the rebuild kit with new bearings and shims to install it. Don't forget you will need to get another carrier for the rear and the front if you go below 4.11. You will also need to change the front to whatever you change the rear to or else you will lose 4wd. I would expect gearing changes for front and rear to run you close to $1000 all-in IF you do the work yourself. If you have someone else do it, add $500-1000 to that depending on your area and the shop you choose. Its a time and labor intensive process to do a gear change. It also requires a press and special tools to properly get everything set up.

The budget and easy way to do this would be to offer up a swap of front/rear axles locally. I bet you could get someone with a set of 3.42 or 3.73 axles from the factory to swap you straight across for those 4.56's... they are desirable for a big tire lifted truck. You can probably find a set of the "undesirable highway geared axles" in a junkyard for pretty cheap too

 

[shiftpro]

I would be looking at taller tires...

 

[shiftpro]

And loosing those rims.

 

[SquareRoot]

Nv4500. Best of both. It would be cost comparative in this situation. Buy once-cry once.

 

[Rusty Nail]

i found out i have 4.56 gears in my 75 k20. i have a 350/465/205 setup i believe. I was told the truck was a dually from factory but now it is a single rear wheel.

What are my options for swapping to a more highway friendly gear ratio? Maybe 3.73? 4.10? I want to cruise at or around 2000rpm going 55-65mph

Any good budget friendly companies that a college student could do? I’m fairly mechanically inclined so i believe I could do it myself.

I would trade you straight for my set of 3.73 axles in a flippin.heartbeat. man....it cant be too hard to find some other willing squarebody guy that lives closer.

 

[Rusty Nail]

And loosing those rims.

Xxxtra hideous!
Deserved of their own post.

 

[shiftpro]

Xxxtra hideous!
Deserved of their own post.

I know I didn't even want to open that but as a decent human man I had to just kinda mention it. He is asking our assistance after all,
.

 

[75k20]

I know I didn't even want to open that but as a decent human man I had to just kinda mention it. He is asking our assistance after all,
.

don’t worry everyone i just traded the wheels away this weekend for some 17” wheels with 33x12.50 tires. previous owner put on the wheels. i also took off the lightbar which they drilled straight through the cab for. i hated this wheels with a passion.

but i decided my best bet was to just stick with what i have and drive 45-50mph on the freeway. the big problem i domt like is getting 6mpg in this thing with commie-fornia gas prices.