Gears or Over Drive

OK I have a F- head in a M38a1 I am trying to bring back to life. I will be putting a T-98 4 speed in this. My dilemma is I would like the jeep to be capable of 60-65 mph when needed.

My two choices 5.38 gears with overdrive (superior in every way except cost.)
4.27 gears Cheap and with granny gear I am still lower than 5.38s and a T-90

The math works out about the same 4.27s = 2690@ 60mph 5.38 w/od 2540 @ 60 mph
I like the way the t-90 drives in town w/ 5.38 gears but it sucks on the freeway. I have never drove a 4 banger with 4.27 gears. My concern is to much of a spread when shifting between gears ( 1st - 2nd ect) with the 4.27 gears. I live in northen Idaho, so lots of hills to contend with.

Any one drive the 4 banger with 4.27 gears? Did you love it or hate it ?
It will be running 7.50 x 16.00 tires w/ 11 inch brakes and Saginaw steering.

any sugestions welcome, Thanks

 

The OD is one of the only mods you can do that you really will get your money back on.

Also it gives you 5.38s when you want it and basicly 4.27 when you want it.

 

THe other great thing about the OD that people dont often realize is the ability to split gears. Say you come to a hill that's too steep for 3rd gear, but 2nd is too low-you can go to 2nd gear OD and keep your speed up much better.

 

x2

It makes the Fhead drive-able in the mountains. Before the OD, 2nd would be too high, and 3rd would be too low. You can always find the right gear for the situation, especially if you put in a T-98.

 

The only way the 4.27 axles would be cheaper than the overdrive is if you already have them. If so, I suggest that you try it out. It would only cost you some time, effort and u-bolts. The overdrive is such an easy add-on that the extra effort shouldn't be an issue.

Realize that you are pushing a lot of air with that F-head, and you can't affect that with gearing. Wind resistance goes up like the square of velocity, so just to overcome the air you need nearly 40% more power to go 65 compared to 55. You'll be running out of poop regardless of the gearing.

 

:iagree: I think 4.27 gears with an F head would be a real dog. I wouldn't even want to go 4.88's really. I'd have to say "good luck" acheiving a consistant 60 mph with an F head unless you only drive downhill. I've been there. Go with the OD, it's much more versatile.

 

this is exactly what I was thinking in my head earlier when I read your post. since Posimoto said it :iagree: as well!!

 

Well the problem is my axle with 4.27 gears is centered not offset I will be rebuilding my rear ends anyway so a gear swap now is no big deal but picking the wrong ratio would be. and I really hate doing things twice.

Yea, thats what I am afraid of too. I was hoping someone who actually drove this set up would chime in. This jeep has a lot of sentimental value to me. and I want it to be fun to drive. But if I can save a thousand dollars (by running 4.27s) I am all for it.

 

I run 4.27s with an F-head and an OD. I can do 60 on the flats, if I hit any kind of incline, I have to take it out of OD. The poor little F head doesn't have enough power to push that much air, as timgr mentioned. The 4.27s are ok for offroad, if you were running a T-90, 5.38s would be better, but if you are going to put in a T-98, it will probably be fine.

 

Thanks Ericm That is what I am looking for. What size tires , any lift on your jeep, Howfast on the flats in direct?
This Jeep would hit 52 mph by the speedo (factory originalfrom 1953) which was calibrated for 7.00-16 tires if speedometer was accuratethen my actual would be about 55-56 mph.granted this was on the flats and the petal on the floor.
it defiantly sounds like the 4.27 are too tall of a gear forthese 4 bangers, especially with the added mass of a t-98 to move around. So I think I will run the 5.38s and startsaving up for the overdrive. I wouldlike to thank everyone for there advice and the information.

 

30 inch tires, 2 1/2 inches of lift. Not sure what the fastest is in direct.

What are you plans for the jeep? The F head is fine offroad. If you need 60-65 on the flats it's possible, but any other time, I'd recommend :v6:

 

Well My plans are a full restoration of the body to Mill spec. But upgrade as much of the drive train as possible with out losing the stock look. I want it ot be a good weekend wheeler (mild to medium) I have a wrangler set up for the rougher stuff. I would like to Drive it to car show and parades and such ( instead of having to trailer it. And use it for a secondary vehicle to run parts etc. (short distance stuff). I live in a small town and everything is 60 miles away so I waold like to get some desent freeway type speed out of it as well. ( I not a big fan off getting Killed because someone slammed in the back of me.)

 

Frank,
My first Jeep had the F134, T90 and 4.27 axles. It would get up to 60 indicated on level highway, but any incline and it slowed down quite a bit. The biggest problem was driving up mountain grades, grinding up in 2nd I couldn't keep up with other Jeeps on runs. When I got on the trail, the 4.27 I thought to be too high geared, that ratio has a pretty poor crawl ratio with the T90C. I would have to clutch, rev and lunge at many of the obstacles and that is really hard on everything.
That Jeep is currently torn down and I plan to put it back together with 5.38 axles, T90 and OD. If I could find a T98 that would be ideal I think.

Don

 

Remember, high gear in a T90 and high gear in a T-18 are the same.

 

after rereading these posts several times. I came up with my brilliant new plan. to do things the opposite of how I usually do them. This will be the first Jeep I have Build were the Body work is a big concern. New Plan Will include looking for an offset D44 with 4.27 gears and 19 spline axles (I will need the axles anyways to run my Posi ) while doing the body restoration, then I can run both sets and make up my mind before I rebuild my rear end. after reading some of the posts about the 4.27 gears I really want to try them out and see how they work with the 4 banger. I have a Centered D44 out of a 68 Commando with 4.27s if anyone wants to swap?

 

Have you considered an engine swap? There are a few modern 4-cylinder engines that are popular in these Jeeps, and one of these would stay fairly close to the stock appearance (as long as you didn't cover it with Chevrolet stickers and such).

 

as soon as i can afford it
OD

 

Here's a wrench for your plans: Have you considered 4.88 gears? From what I've read in this post, it sounds like 4.88'a might solve the slow hill climb problem and gain you a little top speed. It is sort right in between your current choices. That being said, while I put 4.88's in my 5, I haven't been able to drive it much yet. I did notice that I row through the gears in the t14 pretty quick. I run a 225 though vs your 4 banger. I would still like to install the OD. I am not a top speed kinda guy but, with these gears, I can pretty much cruise around all week in 3rd. Ofcourse, we don't have the mountains here that you have. Ehhh, just a thought for you to consider.