Overdrive: Is It Worth It?

is that a valid way to get an idea of the gear ratio?

So yeah that works out to the optional Dauntless ratio of 4.89 gears.
Yes the overdrive is “very” desirable with your gears.

Problem is you need a ten spline overdrive which are relatively few and far between.

 

It's just a matter of giving Advance Adapters some money: https://www.advanceadapters.com/5264

 

I have a '67 with a rebuilt Dauntless, T-86 (with T-90 gears, which have approx. the same ratios), D-18, 3.73 diffs, and 7.00x15 tires that are very close to the stock OEM 29" diameter. And no overdrive.

I do mostly on-road driving, with some light off-road. Though I'm in hilly country, it seems to handle that just fine in the lower gears. I've had it up to 70 mph on the interstate without any problem, as far as the engine and drive train are concerned. However, I also have the stock Ross steering setup, which is pretty twitchy at that speed, and is why I probably won't do that again....

 

My '71 has the 225, T-18 modification and I replaced the main shaft with a 6-spline output during the overhaul (thinking an OD was in the future). I have 4:88 gears and 33" tires.
It's been close to 10 years now, and I haven't sprung for an OD yet.
It's very infrequent that I'm on the highway running over 60 MPH, so I can't really justify it.
I have a car trailer for the longer hauls.
In my opinion, the granny tranny, PS and BDS springs are the top mods for one of these early CJs.
But as you can see, we have strong opinions and good data to back same up!

 

Powerlock inside. Nice!

 

X2
Better axle, worse diff.

 

I wouldn’t hesitate to get an overdrive with 4.88’s. I think you’ll really like the added drivability. I’m running a V6 with a T18, 4.88’s, an OD and 33” tires. It has been a great combo.

 

As was said depends on use and axle gear ratio. I’ve been running 5.38:1 for 35 plus years. I wished I had an overdrive most of that time but never spent the money or trouble to modify my pto winch shafts. My new build is getting an RTS (4 speed trans where 4th gear is overdrive) with a low range conversion in The transfer case (Scout 300) and keeping the 5.38:1 axle gears. It pencils out ok but we’ll see once it hits the trails.
Point is, with lower axle gears an overdrive can be very beneficial. With taller gears like 3.73:1, probably not so much if running 33” or larger tires. If running stock size around 29”, maybe.

 

I have a Ford RTS out of a Bronco...it has a round transfer case mount. How are you mounting it to a Texas pattern?

 

Custom R&P output shaft and steel adapter. We made them for years for Ford Toploader 3 and 4 speeds and overdrives. Mostly SROD’s and the like on the overdrives but others as well. We used to modify RAN 3 speeds to bolt to GM bellhousing and Jeep 18/20 transfer cases. The cool thing is they would bolt in place of a T-90 with GM bolt pattern adapter on front with no driveshaft modifications. We even modified them to use a T-150 shifter top from ‘76-‘79 CJ’s so they were a top shift.

 

I put 66 V 6 3:73 axels in my 64 F 134 when I restored my 1964 Jeep. An overdrive is not needed as I can go 55-60 mph on level roads, with 700 15" tires. I would agree with what everyone is saying too, an overdrive would only help if you had 5:38 gears. Good Luck.

 

Can you post us the RPM's @ 60 or whatever (actual) max speed you are planning to run? This information will answer you question.........

 

I’m not looking to break any speed records or anything. I’m just interested in making the Jeep more driveable at local highway speeds in the 55 to 60 mph range. I refuse to get on the interstate with it and I’m not interested in going any faster than that.

The gearing currently seems to be strained around that speed. I don’t have a tach set up but that might be a good test to see where it falls at that speed.

 

Since you know the final drive ratio it’s easy to calculate the engine rpm at any given speed.
But you must also include the tire circumference into the equation.

 

IMHO, a tach is the first step. This will verify what you are running now without such things as fan and road noise muddling your perception of the engine.

Download a speed app on your cellphone and insure your speedo is accurate. If not, then run you app up to the desired speed and check your RPM's.

Then decide what RPM's you want to run at 60 and see if the OD will drop you within the window you find acceptable.

 

https://www.public.asu.edu/~grover/willys/speed.html
This is a handy speed VS RPM calculator

I have 3.73 axles on my Dauntless equipped CJ5. Around where I live I'm often driving 55 MPH on asphalt. This is comfortable RPM around 2500 with my setup. I wouldn't want to run much faster RPM.
Looks like you would be running 2900/3200 or so at 55/60. I think you would benefit from an overdrive which would drop RPM to 2200/2400.

PS I have this tachometer mounted to the steering column of the Jeep and it works well.
https://a.co/d/iHYLAGM

 

Finally got around to hooking a tach up and driving down the road

I’m hitting 2000 RPM at 40MPH and 2900 RPM at 55MPH.

my tires are 31x10.50x15.

 

I vote You get an overdrive

 

X2