Diff What kind of rear end gearing do I need

[Garys 78CJ5]

I have a 1978 CJ5 with a AMC 304 it has a T-150 Transmission and a day in the 20 Transfer Case I have a Dana 44 in the back and a Dana 30 in the front I would like to be able to go 60 65 down the highway I can't do that right now because the engine is revving too high The gears in the rear I believe are 3.73 what gearing would I need so I would be able to go 60 to 65 down the highway thank you

 

[TSB8C]

Multiple factors will determine your gearing need. Transmission mainly (1:1 in high gear or Overdrive available). You state you have the T-150 3-speed manual, which has a 1:1 ratio in 3rd gear. The next factor is tire size (diameter). Of course larger tires will move you further at a time with each revolution. But of course there is a trade-off in torque as you work to reduce RPMs at higher speeds. Without knowing your tire size, impossible to even make a recommendation. So another factor has to be your main uses for the Jeep. Is this a mainly highway driver, or do you also need offroad abilities?

There are calculators you can reference to determine the best axle gear ratio depending on Transmission gears and tire size. You can then determine the best case for your needs. Mine is mainly a daily driver on the highway in Houston traffic. I have a Transmission with Overdrive that is 0.696:1 in 4th gear. I have 35" tires. So I went with axles with 4.56 gears. My RPMs run about 2200 at 75mph. But this also gives me good torque for off-the-line acceleration in traffic and climbing inclines (freeway overpasses) and pulling a trailer when needed since I have a modest 3.059:1 1st gear. I can also navigate mild trails in the woods when hunting/camping. I don't do any rock climbing or extreme mudding where even more torque may be needed. But I also have an SBC with 420hp and about 400 lb-ft of torque. Your AMC 304 will offer different performance capabilities.

I tried to link a great gear calculator on this site, but the search function is giving an error right now. Here's some other gearing charts for reference though:

Extreme Terrain Link
Interactive Chart
Quadratec Chart
Grimm Jeeper interactive

 

[Jeep Soup]

I’m sure you Know that you will have to Change BOTH axles to the same gear ratio
that you Choose. And you are taking on some work to do so. And expense.

My Nickel.95

 

[TSB8C]

I’m sure you Know that you will have to Change BOTH axles to the same gear ratio
that you Choose. And you are taking on some work to do so. And expense.

My Nickel.95

And some fun

 

[Jeep Soup]

And some fun

For Some of US, mechanical shop work is Therapeutic !!!

My Shop is my Happy Place.
Wasn't offending you,
Just wanted to let you and others know, Axle/Differential rebuilding isn't as easy as those 30 min Videos.
and requires a few extra Tools.

We call it the shop as it is a separate building, 32x88, half Heated shop, office, parts room, and half unheated storage for Toys.

 

[TSB8C]

You are absolutely correct. Axle building does require some special tools and has never been, in my experience, as easy as the videos always show. But usually fun and/or a learning experience nonetheless. Thanks.

 

[Garys 78CJ5]

Thanks for all your info and replies

 

[duffer]

A little late to this party, but 3.73 gears and a AMC 304 , even with no Overdrive , should work pretty well on the highway without over-revving the engine. And why those gears were the standard gear set. 60-65 shouldn't be in any way a problem. In addition to gearing and tires, the engine plays a very big role in this discussion. Something with a mountain of low end torque doesn't need as low of crawl ratio.

If you can't live with the 3.73's, Novak's new Overdrive unit may be what you are looking for. Would appear to be a very robust unit. Not sure where the production of that unit currently stands. In essence, a compact version of AA's former Ranger unit for D18/Dana 20 's.

Official NovaDrive OverDrive Discussion - Novak Forum

The official Novak Conversions Forum — connecting Jeep builders, LS swap enthusiasts, and off-road fabricators to share projects, solve problems, and learn from real-world experience.

www.novak-adapt.com