Jeep transmissions and overdrives

Hi guys

I have a 1975 CJ6 258 I6 3 speed T-14, dana 20.
I'm not real happy with the 3 speed. I find it has a 2nd gear that is too low and a 3rd that is too high. I'm either winding it out in 2nd or lugging it in 3rd going up pretty minor grades. It just does not seem to apply what little power the thing has very well. This is about the only complaint I have with the 3 speed except it might also be nice to have a little more upper end.
A five speed looks like kind of a pain and more work shifting than its worth and this thing is not a Ferrari.
I'm ignorant about Jeep transmissions, overdrives and gear sets and what is available. I realize that I may have formed some incorrect impressions and want to learn much more before deciding what to do.
I've been looking at gear ratios in T18s and for my type of driving, the low is almost useless and not synchronized anyway. The wide ratio T-18 being the most useless (to me) and the narrow ratio somewhat better for my type of driving but I hear they are hard to find.
I've looked at overdrives briefly but so far I can only find old stuff that is not compatible with a dana 20.
The articles I have seen also mention that some overdrives were not a good idea for short wheelbase Jeeps but I don't have that particular problem.
I guess where I am going with this is, I like the simplicity of adding an overdrive rather than replacing the transmission but have no idea if it is possible without major expense and hassle.
I've been looking at Herm the Overdrive Guy's site and some of his stuff looks pretty cool but I don't think I know enough to buy intelligently.
Does anyone have any opinions or advice?
I'd also like to hear from someone that owns a 4 (or 5) speed CJ6 258 I6 that has driven a 3 speed and/or upgraded to a 4 (or 5) speed or overdrive to see whether they thought it was worth it or if they would rather have done something else.

Thanks for any help you can give me on this.

Jon S.

 

I've owned a CJ-6 similar to yours: 258. T-14 3-speed. You're right, the ratios in the T-14 are awful, for the reasons stated.

Look at the ratios for the T-14: 3.10, 2.61, 1.

A comparatively easy change would be to remove the T-14 and substitute the Borg-Warner T-15 3-speed from a '72-75 V8 CJ, or a Tremac T-150 3-speed, from a '76-79 CJ. Both of these transmissions have more evenly divided ratios than the T-14.

T-15: 3, 1.83, 1 and T-150: 2.99, 1.75, 1 ... much better ratios IMO.

<edit> Also, what's your axles ratio? Neither of the available factory axle ratios (3.73 standard, 4.27 optional) need an overdrive IMO. The 4.27 ratios will give you more power, and the 3.73 ratio will give you plenty of top speed.

 

Tim

Thanks for your reply. BTW, I have already installed that CRT Performance distributor and am really liking it and the new Weber 32/36 DGEV K550 should be arriving today. Can't wait to get rid of that old YF turd.
Maybe I won't have to smell gas while pounding over the landscape anymore.

To answer your question about the axles, I've never looked at them but I think they must be the 3.73s.
I'm running 32.5" Super Swamper tires on 15 inch rims and I can get it going about 60 or 65 in 3rd.
I think the 3.73s are probably better for me since I don't trailer my Jeep and I am not really into rock crawling. People tailgate me as it is because I refuse to beat the thing up just to try to keep up with the BMWs.
This is off subject but I'll never forget towing the Jeep home (55 mph) behind my Toyota pickup on I5 and watching too many people approaching my rear at well over a hundred and veering into the fast lane approximately six and a half inches from my bumper. Gave me an idea what I was in for. 4.27s could be fatal on road in California. )

Back on subject, I guess an overdrive isn't a viable option with what I have so a different tranny is indicated.
I read what you said about the other 3 speed ratios. If I'm reading that right, you would prefer the three speed to a four speed?
I've been looking at Herm's toy store and there are some interesting ratios in there. Are you familiar with the Ford RTS w/OD? Cool ratios but I don't think I know enough about it to wing it and try to find or buy one.

Ford RTS Truck style 4 speed OD transmission - $1,150

This transmission is called an RTS. The RTS transmission is an alternative replacement as a heavy duty transmission with overdrive for D-18 and D-20 transfer cases. It can be used in conjunction with a PTO (WARN) style OD as well. When used this way, the RTS gives a 43% increase in speed and a 43% reduction in RPM.

It is a heavy duty transmission originally installed in Ford pickups and Ford Broncos. It has an aluminum case with an aluminum shift tower with a cane shifter.

The RTS has large heavy duty bearings, is completely rebuilt and comes with an adapter for a D-18 or a D-20 transfer case.

This transmission is a good replacement for the T-14 transmission behind a V-6. This application requires a special adapter and pilot bushing to bolt to the GM bellhousing which is available separately.

The RTS is also a direct replacement for the T-150 transmission ('76-'80 CJ's) that bolts direct to the 258 6 cylinder(4 liter) and 304 V-8 with the D-20 transfer case.

The RTS is also adaptable the the '72-'75 258 6 cylinder and 304 V-8 with the changing of the bellhousing with the D-20 transfer case.

1st Gear 3.25 to 1
2nd Gear 1.92 to 1
3rd Gear 1.00 to 1
4th Gear 0.78 to 1
Reverse 3.25 to 1



What do you think about this RTS with OD? I wonder if the 258 has enough power to use that 4th gear on flat ground or actually even going uphill.

Thanks again for all of your help.

Jon S.

 

To answer your question concerning enough power, you must analyze the horsepower/torque chart for the engine..
Using your gearing and tire size, determine how many rpm's the engine is turning at various speeds.
There are various sites available where you can plug in these numbers.
You want to be running within the "power band".
I had experience with '86 F-150 with OD transmission. 302 FI 3:55 rear. Lots of go until you hit the OD.
Too much of a gap in ratio. OD was only good for flat running. New ones probably better.

 

Well, I owned a '75 CJ-6 with 258 PS/PB and 3.73 axles in '75-76-77 when I lived in CA. That Jeep had a steel hard top and 31" tires... I had no trouble at all driving the freeways of CA. I got speeding tickets in that Jeep at least once that I can remember - maybe more. I owned a '73 with a 304 and 4.27 axles before that, and similar tires. Again, I had no trouble driving above the speed limit on CA highways. Both of these Jeeps went back and forth to the desert and local mountains dozens of times under their own power. Both were driven to the Grand Canyon and back several times.

IMO adding an overdrive gear will make your highway power problems worse, not better. Power is proportional to torque times RPM. If the torque curve were flat (it's not, but let's pretend it is), you'd get twice the power at 1000 rpm that you would at 500, and again it would double at 2000, and again it would double at 4000. Higher engine speed makes more power. Let's say you have 33" tires and 3.73 gears. In top gear, you'll be going 3418 RPM at 90 miles an hour... that's a comfortable speed for the engine, and should be fast enough. With 4.27s, you could go about 80 mph for the same RPM. Not a problem. May be loud, but not a problem.

You see my point? I suggest you get a tach and see how fast your engine is really going at highway speeds.

Re the three speed, you were complaining about the gap between 2nd and 3rd. That's a problem with the T-14. You can fit a transmission with less of a gap (T-15 or T-150) and you'll gain a usable passing gear. That's all I was suggesting.

 

In my 74 CJ-5 I run a T-176 4-speed that was used in CJ's from 1980 onward. It wouldn't work with your D20 though, which is a bummer. I got a very good deal at the time I swapped it on the entire drivetrain (engine, tranny and t-case) from a 1980 CJ-5 so I just swapped in the whole setup and went with the low mileage 258 and D-300 t-case. I really like this combination. The gear ratios in the T-176 are very friendly to daily driving. It doesn't have a super low 1st gear, but it's not bad. It was also used in Ford trucks of the same era.

If you do swap, avoid the weak 4 and 5 speeds used in the 80's, like the SR4 and T4 and T5. They are usually fine for stock setups but fail at an alarmingly high rate with any amount of extra stress.

 

I think that a Dana 300 would be a plus!

 

I would do nothing till you have installed and tuned the new carb you have coming.
I think the 32.5 tires mixed with the 3.73s are what are hurting you.
report back after the carb install; either way (gears or trans) it could be expensive.

 

I think a T-176 swap would be a big improvement over the T-14. They are reputed to have a couple of problems though ... the shift tops wear out and then the transmission will jam in gear. Replacement tops are available aftermarket. They also have a reputation for breaking the main shaft where the shaft necks down to accommodate the output gear. Not as strong as a T-18 or such, but I'd guess they are roughly equivalent to the T-14 in strength, and would be a good match for the 258. Driveability would be similar to a passenger car 4-speed.

 

Tim

I've just been lining up what little things I need to buy or modify on the new Weber and right off the bat the manifold adapter doesn't fit properly onto the manifold studs because they are too long and they hit the casting in the middle at the constriction point. Also, when I looked at the casting, it looked like someone milled out the carb side flange holes. It kind of looks like it might have been used (abused) and modified because whatever wouldn't fit. Not real happy about this but not sure it matters at this point.
I don't want to have to grind down those studs. Wondering if I can relieve some material on the casting without breaking through.
Does it look to you like someone messed with this casting?

 

Tim

Its hard to estimate how much material is left on the casting on that constriction wall.
I don't want to have to remove and cut or grind those studs.
Kind of ****es me off I have to do this.
Most of my tools are about 150 miles from where the Jeep is. Oh well. Best laid plans and all that stuff.

 

Mine looked just the same. A little cobbly but works fine on my Fhead. It's designed to be a generic adapter. Just be mindful of sealing air leaks.
Your studs can easily be replaced with shorter ones.

 

Yeah Homebrew I hear you.
Unfortunately, my stud puller is far away from here and I learned vice grips can rapidly make things much worse.
I brought a ton of stuff to make sure I had everything I needed and I wasn't close enough.
I'm just whining because I wanted to go wheeling this weekend and now it looks like another 300 mile round trip just to get the right parts and tools together. I'm missing some other stuff I need too. )
This is just like last time when I installed that CRT distributor. Left my timing light at home like an idiot and had to buy a new one.
Always something.

 

Oh, sorry you have other stuff you need too. I used two nuts to remove my studs. I was just thrilled I didn't have to drill and tap one of them like my exhaust man. studs

 

Another good source to look at is partsmike.com. Call him for advise. He has great experience with the t-18's and would be worth talking to. I will be going to a t-18 with the 6.32 first gear in my 67 but I need it for the rocks.
Peter

 

Thumbs up to the T-176/Dana 300 swap. I swapped this combo in behind my Dauntless and I love it. I don't rock crawl so I don't need the ultra low first gear. For mud/trails/hills this is a great set up. Dana 300 low range is pretty good for a stock unit. Although, if I had the wheel base of your 6 I might consider an automatic if you do most of your driving on the black top, just a thought. I hate automatics....

 

I run 4.56 gears in my 74. Has 35X12.5X15 ProComp Mud terrains on it. They are very close in size the the 33X12.4X15 BIAS TSLs I had on my other jeep. With the 4.56 gears it would run down the highway at 70 without too much trouble. The temp would raise to about 200°. Gas mileage suffered tremendously over 55 or so.

With the 3.73 gears, same tires, I would run it down the highway 75mph all day long. At 65 it was really happy and got 20mpg.