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recommended gear ratio

wow..... that's a TOTALLY new subject!!!!!!

hahahahaha

Search... It's been covered a hundered billion zillion times.....
 
Crunch the numbers, and think about the purpose of your rig.

Typically, you want to set up so that you end up cruising around 3000rpm at whatever speed you use most (default: freeway speed.)

If it's a "trailer only" rig, go as deep as you can.

But, you're honestly the only one who can make that sort of decision. That's why everyone tells you to search - gather information, think about what you're trying to accomplish, and then go forth with that knowledge.

Just about every rig here is unique, and built with a certain purpose (or purposes) in mind. Just like yours...
 
5-90 said:
Typically, you want to set up so that you end up cruising around 3000rpm at whatever speed you use most (default: freeway speed.)
Click to expand...
While I don't think I've ever questioned anything you've ever posted, that sounds high. I thought that, with stock 215/75/15 or 225/70/15 tires (only a .3" difference between them), the RPMs would be around 2100 at 65 m.p.h. with the 4.0L and AW4 combo. I understand that we don't want the tranny lugging along at too-low RPMs.
 
There are many factors that will decide what is the optimum gear ratio for a given vehicle. Starting with how much power the motor puts out and running tha gaumut to the type of terrain most often traveled. From some of your other posts I have noticed that you are building a trail only rig with an atlas. With that and a set of 35's Personally I would go with the 4.88's but that would be for the terrain I deal with. Also there are many options with the atlas for the low range ratio from 2.73 to 6.0 iirc so there can be some steep gearing without having to go crazy in the axles since you are going that route.
 
99XJWhitey said:
What is your exact setup and what will you be using it for? Lets start there.
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Yea that is a good starting point. Usually deeper is better for off roading. Especially when you want control over wheel speed. If you want wheel speed, ie mudin, then you dont want to go deep like you would for say rock crawling.
 
exactly. I had my wrangler running 35's with 4.10 gears because I frequently drove it on the road and it was right where I wanted it. But, my XJ is running 35's with 5.13 gears. it never sees the highway. When I bought it, it already had this setup on it and a rod thrown through the block if that gives you any indication on what the RPMs were at cruising speed. haha.
 
Timber said:
While I don't think I've ever questioned anything you've ever posted, that sounds high. I thought that, with stock 215/75/15 or 225/70/15 tires (only a .3" difference between them), the RPMs would be around 2100 at 65 m.p.h. with the 4.0L and AW4 combo. I understand that we don't want the tranny lugging along at too-low RPMs.
Click to expand...

"Around" is a little vague - I think I usually cruise at 2700-2800 (88/3.07/4th/30's.) I still want to re-gear one of these days, but I've had larger things to do.

If you search up, there are a couple of threads on engine efficiency vice crankshaft speed, and it's not just me saying this. I've taken the time to verify it (as much as possible) experimentally, but experiences seem to have shown that this is a good target for cruising speeds.
 
5-90 said:
"Around" is a little vague - I think I usually cruise at 2700-2800 (88/3.07/4th/30's.) I still want to re-gear one of these days, but I've had larger things to do.

If you search up, there are a couple of threads on engine efficiency vice crankshaft speed, and it's not just me saying this. I've taken the time to verify it (as much as possible) experimentally, but experiences seem to have shown that this is a good target for cruising speeds.
Click to expand...
Very good, then. May I then surmise that the reason Jeep was keeping the RPMs lower was almost strictly for better fuel efficiency?
EDIT: Sorry, not trying to hijack this thread.
 
Timber said:
Very good, then. May I then surmise that the reason Jeep was keeping the RPMs lower was almost strictly for better fuel efficiency?
EDIT: Sorry, not trying to hijack this thread.
Click to expand...

I'm not entirely sure. An engine is most efficient at the point where it produces peak torque and experience peak VE - it falls off anywhere other than that. Fortunately, our inlines tend to give us a nice, flat torque curve - 90% or better from off-idle to about 4000rpm or so (with a peak somewhere just south of 3000.)

It's like a piston aircraft engine - you set your cruise crankshaft speed to match peak torque, and you'll get greater range. I tried not using fifth gear (3.07 and 31's at the time) for a while, and the increase in engine speed at that point was worth about 4-5mpg!

I'm not sure of the physics of it all, so here comes the guess:

I think it's the overall efficiency of the engine at that point that has a lot to do with it. It probably doesn't need all that torque to maintain the speed when cruising (even if our rigs are about as aerodynamic as cardboard boxes...) so the greater the ability of the engine to make more torque than it needs, the less fuel it will need to make the torque it actually does need at the time. At least, that makes sense to me (research into the subject hasn't uncovered the answer. Yet.)

Now that that's out of the way - I know that most automakers are gearing and setting up to cruise on the freeway (or wherever) at as low a crankshaft speed as possible - and I think that's a mistake. Cruising a little higher may seem counterproductive, but experimentation has borne the idea out as useful. I think it's a marketing thing, more than anything else (and has a lot t do with why light truck engines are marketed by horsepower, rather than torque output. Believe me, you get much more use out of torque in your truck than you do horsepower. Torque gets you moving off the line, makes you more nimble, and lets you pull loads. Look at any dyno chart to see what I mean - horsepower doesn't usually start picking up until somewhere around 3000rpm or so, and it's still on the way up. The torque output is significantly higher at that point. HP doesn't exceed torque until 5252rpm.)
 
I concur on the 4.0L loving the 2500 neighborhood. When I drive on the highway for any length of time, I get 22+ mpg with 4.88s and 36" tires!!
 
XBoBJ said:
how do you know all this stuff 5-90 your a jeep connoisseur
Click to expand...

Research and a widely varied experience. It would take rather long to list everything I've done (professionally or as a hobby,) and I really don't want to go into it.

Much of the rest is figured out through some rather educated guesses and as a "student of human behaviour" and a people watcher. I've learned more about psychology and sociology that way than I did actually taking uni-level courses (and it's more practical knowledge that can be applied to other fields.)

I am also hampered by "free association" - at random intervals, a number of facts and observations come together for no apparent reason and synthesise into an effective conclusion. This very much has its pros and cons - you can get revelations without thinking about things - but they're not always germane to whatever you're doing at the time. And, write them down before you lose them (I just about always have a notepad in a pocket somewhere.)

I'd like to put it to use - how about instead of all these studies that various people commission (and spend millions of dollars and several months on,) you just drop by with a "consultation fee" of $5,000-10,000 and give me your question and 48 hours? It will cost a good deal less, it won't be any less valid (if you feel like it, you can test my conclusion afterwards,) and it will be a good deal quicker...
 
I'll be your consultant for say..... 30%?:eyes:
 
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