Overdrive.

What is the difference between the 25% Warn overdrive and the 30% overdrive? Which one do you like?

 

5%

 

That was funny. I've only heard of the 25% model and they are incredible. My 4.88s drove me nuts above 50 mph, now with the equivalent 3.55s, I can go anywhere.

 

I have only heard of the old warn 25% OD. There was a OD that was used in Studebakers, but don't think you can or would want to use one. There is a company making recopies of the old warn unit, maybe they added 5% more reduction to it, I know who makes it but cant think of it to save my life right now. It,s 3AM and I couldn't sleep, so im a bit tired.

 

advanced adapters made mine got it from walck's

 

According to my math, 4.88's with a 30% overdrive would be the equivalent of a 3.42 gear.
With a 25% reduction, it would be 3.66 (real close to the stock 3.73)
The same math with 5.38's comes out to 3.77 or 4.04.
My opinion-if I had 5.38's and stock-ish sized tires (28-30" range), and an engine with more power than an f-head, I'd consider the 30% unit.
Otherwise, I think the 25% unit would be better.
One of the other benefits of an OD, besides the improvement in top speed, is the ability to split gears (1st gear OD, 2nd gear OD etc.)
I think the 25% unit would offer more even splits than 30%.
Please note that this is all theoretical-I've driven a ton of miles in several Jeeps with the :v6:, 4.88, Warn OD combo, but I've never driven anything with a 30% OD.

 

Old Time wrote a great post on this a while back. While I don't know where it was, I seem to recall the consensus in that post was that a 30% O/D is really only gonna be useful for final drive. Where as with a 25% you can split gears and still use it for cruising in 3rd (or 4th).

 

Scott, with the 4.3, you could make good use of a 30% unit on-road but a 25% may work better with the 4.27 gears all around. I run a 25% with 4.10's and have no complaints. The mileage difference would likely be negligible. Either will work well at freeway speed.

 

You can read my opinion here: http://z4.invisionfree.com/CJ3B_Bulletin_Board/index.php?showtopic=4317

 

Ken,

thank you for this!

 

Here are the mathmatically perfect split ratios for use with 25% and with 30% O.D.'s.
Compare your known transmission gear ratios with these numbers.

25% OD = .75 in 3rd +OD > 1.00 in 3rd > 1.33 in 2nd +OD > 1.77 in 2nd > 2.37 in 1st +OD > 3.16 in 1st > 4.21 in low +OD > 5.62 in low

30% OD = .70 in 3rd +OD > 1.00 in 3rd > 1.43 in 2nd +OD > 2.04 in 2nd > 2.92 in 1st +OD > 4.16 in 1st > 5.95 in low +OD > 8.50 in low

Note: truck type 4 speeds have an extra gear that I prefer to call low gear.

 

Going to pick up the overdrive tomorrow. I think anyone of those overdrives will work. I just want to extend the range on the Jeep. 11 gallons doesn't go that far. I think with the fuel injection and the overdrive I should do pretty good.

 

An early 70's rear tank is an easy add on & adds 15 gallons...

 

There are lots of comments and calculations regarding final drive ratios of 4.10, 4.27 and 4.88 with the 35% and 30 % ODs. What about 5.38 gears? How do the two overdrives compare? That's with the F-head.

 

To Scott Milliner -

If I may throw in my totally subjective opinion about this -

Since you have a modern engine and 4.27 gears, I would question whether you need an overdrive at all. At a true 65 mph, you're only turning 2800 RPM. I kinda think you don't want to travel much faster than that in a factory chassis M38A1. With the 25% overdrive, your cruising RPM drops to 2100. Subjectively, I would not see much advantage to lowering the cruising RPMs much lower than that.

Again, subjectively and with no personal experience, I think the gear-splitting capability is a little overrated for how much it costs you. With a truck 4-speed, you already have a wider selection of ratios than you would with a factory-equipped Jeep. Having the overdrive on the highway may be advantageous if the engine and gear noise is really bothersome, but I suspect I would not do a lot of overdrive shifting off the pavement.

 

There's been a lot of discussion about the F134, axle ratios and overdrive in the past. With the F134, you have the added issue of running out of power. For example, if you decrease the final drive ratio to 3.54, the Jeep should have a lower top speed than it had with the factory 4.27 or 5.38 gears. Recall that power is torque x RPM, so if the torque curve is flat, the engine makes more and more power as the RPM increases.

There is also air resistance - which goes like the SQUARE of velocity. So air resistance at 60 MPH is four times what it is at 30 MPH. This is a mighty barrier, and is especially limiting for an aerodynamically terrible vehicle like a Jeep.

You are also RPM limited with a loooong stroke engine like the 134. Willys didn't even test the engine above 4000 RPM.

I think the consensus was that the 25% overdrive with 5.38s is ok, and will give you some amount of additional top speed under the right conditions (ie flat, no head wind).