Dauntless Overdrive And Axle Ratio

Hello,
I'm putting together a CJ5 with a 225 and 3.73 axles. The tires are 225/75r15 which are a hair over 28". The goal for the vehicle is that it be able to sustain 60 mph on flat asphalt roads comfortably. Playing around with speed calculators that should yield about 2800 RPM. A 75% overdrive would be right about 2000 RPM. Would an overdrive be asking too much of the engine? I'm not sure if that's too much horsepower required for that low of an RPM. I think less engine noise and vibration would be nice if it's feasible. Any thoughts are appreciated.
Thanks,
Ken

 

In my mind D20 is superior to D18 when Jeep is intended toward lots of of hiway miles between the off road adventures. For one thing it is quiet.
A D20 cannot accept an overdrive so it is best to gear the axles with 3.73 or 3.54 ratios just like Jeep did it.
The engine rpm will be about right for most engines if running 3.73 /3.54’s.
You can fine tune the exact engine rpm via tire diameter.

For a couple reasons I consider the D18 better for increased off-road use.
For one thing the axle differential carriers will be in-line. With a D18 axle ratios can be much slower like 4.89 or even 5.38 because a 25% overdrive is possible. D18 / 4.89 / 25% overdrive is an excellent combination allowing good hiway operations plus excellent crawl speed. Here again you can find tune the engine rpm to specific speed via tire diameter.

Myself would never combine 3.73 to overdrive. In fact in 1964 when the Warn overdrive became a genuine factory approved accessory; Kaiser Jeep Corporation stated that it was only to be used on jeeps having 5.38 or 4.88 axle ratios.

 

With my setup I am about 2050 rpm at 60 mph in high gear. I have no problem cruising along nice and smooth, acceleration up to 70 is easy.
I however am not running an overdrive. I am running an D20 and centered rear diff.
With 3.73 gears you are probably better off throwing some 31x10.50 tires on and running it.

 

I found the 4.89 gear option with the 225 to be just about perfect with a Warn OD in my 67 CJ5. No problem going 70 down the highway and it actually got decent mileage-usually around 18 mpg.

 

In my experience, an overdrive would go unused with 3.73s with a 31”+ tire.

28” tires and 4.10+ gearing would benefit from an OD.

I feel like you’re right at that breaking point where either with OD or without will be fine, but neither are perfect.

I’m a fan of less complexity, so I’d go without the OD with your setup. The 225 won’t have any issues at 2800.

 

Just wait till you see how it preforms with fuel injection....
I have no idea what cam is in my engine. I know that it was rebuilt prior to my taking ownership and it pulls good from idle on up.
My drivetrain is built around a cruise speed of 70mph @ about 2400 rpm.

If the OP's jeep were mine I would go to a larger tire for the time being and run it as is. Tires will cost the same as an OD....
This jeep hobby has a way of creeping out of control.
I spent a bunch of money on overdrive, axles, powerloc's ect when I was way too green in this hobby. I read lots of forum posts and thought that was what I had to have.......
After a couple years with the "good ole type setup" I tore everything out and completely reworked the drivetrain and am much happier now.
Unless it's a person's second or third build.... I cant stress enough the need to "run it" for a little while.

 

Thanks for the replies. Perhaps larger tires would have been a good idea but I was fitting them to original 15-4.5 wheels so I wanted to err on the smaller side.
I got an overdrive yesterday as part of an engine / transmission package so I was hoping I could try it out. Unfortunately it turned out to be a 6 spline unit and I need a 10 spline for my T14. (the transmission in the package was a Ford toploader with an adapter so I didn't know what type it was).

What do do, what to do.......

 

I'm another one who's a fan of the 225/od/4.88 combo...great all around performance in a CJ.

 

For quite a few years, I ran a 225/T18 with 3.73 gears and 31” tires. This was a nice combo; crawled well and ran down the road nicely too. Since then, I changed to 4.88s with an OD and 33” tires. This is a great combo too. Allowed for better crawling off road, while maintaining good highway manners. I like the OD’s ability to split gears too, adding to the flexibility of this combo.

 

In my opinion, pairing an overdrive with 3.73 gears is all about the show of an extra shift lever sprouting from the floor. It really has no practical use.
I ran 4.27 gears in my Jeep with 29" tires and a 215 Buick V8. I ran an overdrive in it for a year or so. All my on-road driving was on virtually flat ground. Though the engine pulled the gearing, it was only really useful at around 70 mph and up, not recommended for a short wheelbase Jeep. The overdrive did enable me to wind my speedo needle past the 9 once, but I was a lot younger then. Modern engines (1990s-up) are designed to make low-rpm torque where they run between 1800-2100rpm out on the road for fuel economy. The Dauntless is happier spinning 2500-3000 out on the road.
3.73 gears are well suited for V6 (or V8) power and no overdrive. Since you also need the 10 spline version, I would either re-gear to 4.88s for use with the overdrive, or shelve/sell the overdrive.
-Donny

 

Thanks everyone for the thoughts. I certainly don't want unnecessary complexity. 2700/2800 seemed fast to me based on the other vehicles I drive (F134), but if the Dauntless is happy doing that continuous then I guess I'm set. Tomorrow I'll install the tach in it and see how it sounds at those speeds.

 

So you were all right. I checked my speedometer against a GPS and it was pretty accurate. Cruising about 60 yielded around 2600-2700 RPM, the engine sounded very reasonable. I don't see anything to gain with an overdrive.

 

iv got 31s and no od and its no problem. I dont run over over 65 anyway it gets sketchy especially if its at all windy feels like the force on the windshield makes the front end quite light.

 

This is what I run with 33" tall tires. It seems perfect for me.