is there a down fall I forgot?

I have a 70 CJ5 that I am considering doing a little bit of swaping.

it currently has V6 225 with a T86 and spicer 18 with 4.88 ring and pinion front and rear.
And I am going to go with 33" tires when I finish buttoning her back up...currently a project Jeep.

I want to use it as a daily driver and toy...the down fall is the gearing for highway right now has the engine reving at approximately 3300 RPM to go 65....

to make it more fuel efficient and still a fun weekend toy I am thinkning of going to a SM420 tranny (still gives me a crawling ratio) and changing the ring and pinion to 3.54 front and rear.
this will allow a good crawling gear, but at highway 2300 RPM to go 65

The downfall that I know of is I will have to do some driveshaft adjustments as the tranny is slightly longer then the factory tranny and then the adapter will also make the transfercase even further back.

Is there anything other drawbacks?

 

The only thing I can think of is would you have enough power at 2300rpm to push the Jeep at 65 with a fuel efficent percentage of throttle? I am not a physicist nor did I stay at a holiday-in express last night so I don't know if that would be a problem or not.

edit: I like Sparky's idea better, especially the first one, mucho easier compaired to the swap.

 

I would get the od 1st, then see if you need the sm420 (picked up about 3.5 mpg with mine). I don't think the :v6: would like 33's with the 3.54 gears at all.

 

R)

tommy b

 

I have the sm465, 4:88's and OD behind a 2 bbl V-6. The best I have gotten is 24mpg on a road trip. The OD lets me cruise all day at 65 mph around 2400 rpm. I would go with Sparky.

 

I would go with a warn OD first to reduce highway rpms. They work great.. most of them have a gearing output of 0.75. I think some older ones are 0.70. You will really want your 4.88's when climbing up winding mountain roads to get to your destinations. And, I dont think 3300 rpms at 65 mph isnt that bad at all.. and you probably dont want to go much faster than 65 in your jeep ?

This .75 should bring you to 2500 rpm at 65mph.

 

You never said what size tire you have right now. 33's as well? If anything shorter, the switch to 33's will lower RPM's as well.

 

Your V6 will be happy at 3300 rpm running all day long, so it's not really a problem unless it bugs you. If you aren't used to it, the Jeeps (esp. with a D18 and no floor coverings) make a lot of road and gear noise. Also, a truck 4-speed is going to be considerably noisier than your 3-speed transmission. No experience with the SM420, but I'd expect it to be pretty loud considering how old a design it is.

The 3.54 gears and the overdrive would both bring you to about the same rpm at cruising speed. However, adding an overdrive will be cheaper than regearing, it will add resale value to the Jeep (which 3.54 gearing will not), and if you don't like it, you can remove it and sell it for nearly what you paid for it.

 

Overdrive, :iagree: Call up Herm the overdrive guy, he can help you out, or look around, you might find a used one but there rather scarce and seem to go for alot of coin these days.. I think you need one with 10 spline 29 tooth gear to run with your stock set up. This is an odd ball size. If you swap in a 4 speed, all the adapter shafts are 6 spline, so you will then need to get a diff. bull gear (6 spline 29 tooth)on the overdrive which is expensive later. So decide before you get an overdrive what you want in the long run. I love my overdrive and have swapped in a 4 speeds in both my projects and really love the combo.

 

where'd ya pick it up at?

 

Thank you all for the input.

This is my first real project...tear down to rolling chasis and then rebuild.
I've had a 67 bronco that needed a lot of work but never took the body off or anyhting that intrusive.

had two other jeeps but again no tear down just bolt on and lifts.
I really want this to be one I hang on to and one that I use all the time...so being able to go 65 below 3grand RPM would be useful gas wise, and still be able to take it on some trails would rock!

 

It came with the OD when I bought it, PO did not know what it was. He said for some reason it was hard to get going in 1st gear (t-14) I picked up the 465 from a friend for $50.

 

He's running a T-86 so that would be 6 spline. Only T-14's are 10 spline.