Choosing the right gears for your Jeep....

[Overland History]

I haven't seen a thread like this within this forum thus far, so I figured I'd start one. This thread is stemming from a recent thread on my response to a thread here.

http://www.expeditionportal.com/forum/threads/154440-Gear-and-Locker-Question

I'm quoting myself from this thread, because I think the discussion needs to be opened to educate those that are planning on lifting/gearing their Jeeps in the future.

Look, gear ratios are largely opinion and highly debated.

With a simple Google search, find out and punch in all of the specs to make sure you don't make an expensive decision that you'll end up regretting.

This website may help....

http://www.grimmjeeper.com/gears.html

The main problem that everyone seems to worry about is engine RPM on the highway, which is largely one of the more irrelevant factors when it comes to choosing gears.

If you have an automatic transmission, the main and HIDDEN factor is heat! When you add larger tires and more weight to a vehicle, you put more torque on the transmission! Auto trannies hate heat and don't survive long under those conditions.

If you have a manual trans, you wear out the clutch faster due to more torque being necessary to get the jeep moving.

Think of it like this: Your Jeep is a 12 speed bicycle. Your stock gears and bigger tires are similar to kicking the bike up to the 12th gear and trying to go from 0-60mph. Now also add a heavy backpack on your back(which are your heavier tires, bumpers, rack, winch, RTT, gear, etc.). Your body is the transmission. How much are you gonna sweat trying to pedal all of that torque and weight up to speed?

The point of gearing is to OVER compensate for all of the torque and weight that you've added to your vehicle!

With that said, I don't know JK's that well, but I would be interested to read a discussion on what works for the owners with consideration to the mathematics of ratios!

On my '92 XJ with a 4.7L Stroker motor, I had 4.88 gears and 33in tires. I LOVED the gearing, especially off-road. It would shift into overdrive in high at 38mph. In low range, I could just about feel the lumps in the cam. The reason I went with super low gears is because I love to crawl off-road, but I also wanted to compensate for the weight that I added to the Jeep. Even with the Stroker motor and gears, I still got 18mpg on the highway.

Discussion is open for opinions, but ultimately physics will win!

 

[tarditi]

Agreed - it is an equation with a LOT of variables and influencing factors.

A couple additional things to consider:

1) manual vs auto (torque converter can compensate for some inefficient gearing, but at the expense of heat (as you mentioned))
2) Final drive ratio in transmission
3) transfer case gear reduction
4) Torque of engine
5) Driving habits
6) Vehicle weight
7) Expectations

For instance, I have a 2015 Jeep Wrangler Unlimited Rubicon - softtop, manual trans, stock with just a few bolt-ons.
The engine is quite torquey compared to the older 3.8L JK engine, 4.11 gears and the 4:1 xfer case makes crawling a very slow and deliberate activity. On the highway in 6th gear I can cruise at 70mph around 2200rpm and still get 17-18mpg. I plan to go to 32s or 33s in a little while and expect a slight degradation, but not too bad.

I used to have a 1997 TJ Sport with 4.0L engine and 3spd auto trans. With 33" BFG MTs and a 3" lift, and few other things like armor, bumpers, winch, etc. I was getting about 11mpg and the vehicle would about overheat on the beach (hot weather + partially obstructed airflow from winch + resistance from sand).

 

[Happy Joe]

Over the years, and many vehicles/gear setups, I found that the physics and engineering does affect the optimum gearing.
It all starts with the engine torque curve; setting the gearing so that high way cruise is near the peak of of the torque curve (if it peaks out at a useable rpm (usually ranging from 2300 to 2800 rpm) seems to give the best highway mileage which is very important in a dual purpose machine. (over about 2800 or 2900 rpm and the engine life seems to be progressively reduced). It should be noted that virtually all of the hot rod engines that I did saw an increase in fuel mileage; as long as I could keep my foot out of it.

Given that the engine torque curve doesn't change all that much unless you modify the engine the thing that most people do that affect s the overall performance of the vehicle the most is to change the tire size. A larger tire diameter tire takes more torque to turn it down the road (mostly on starting) This then affects the gear choice to keep the engine rpm in an acceptable mileage range and the over all drivability acceptable.
The tire weight, width and air pressure, indeed even the wheel width enters in to mileage also; a skinny large diameter gives the least rolling resistance when inflated to a high air pressure; but not the best traction, ride nor life.

Poor tire/gear selection results in poor or at least reduced drivability.
I found that for a light vehicle (CJ) with a non granny low manual transmission and 33 inch tires 4.27 gears worked the best overall; 4.11s were a bit too high on and off road, 4.56 gears required excessive highway rpm and 4.88s were even worse.
When I went to 35s, overdrive became unusable with the 4.27s and 4.56s were the best choice.

Anyone that slips the clutch isn't driving the vehicle correctly, although extremely poorly set up vehicles will push people toward this practice.

When we go off road and shift into low range things change a bit since few really look for or care about mileage at this point.
It is my firm conviction that off road gearing needs to be addressed in the transfer case.
Crawl ratio is the total lowest gearing of the vehicle, and lower is usually better until you pass 70 or 80:1 (find this for manuals by multiplying; axle ratio x transfer case low range x first gear; for automatics do the same but multiply the total by around 2 to approximate the slippage in the torque converter when going very slow).
You hear about people who brag on 100 or even 200:1 crawl ratios in my experience these are largely unusable off road; when you get to even 60 or 70:1 you basically eliminate being able to give an obstacle a momentum bump in first gear (besides being incredibly boring to drive). So far the best traction terrain that I have encountered is in Moab UT even driving this sand paper like surface very low crawl ratios are often unusable the tires just slowly spin. Heating the tires up can give better traction but extreme crawl ratios mean that this takes extreme rpm.

Automatics are often selected by rock crawlers to reduce the number and frequency of driver decisions that need to be made off road. The slippage in the automatic reduces the craw ratio and an increase in rpm reduces this slippage as the torque converter locks up. The slippage also seems to cushion drivetrain shock loads to an extent. They have their own issues starting (as mentioned) with heat few hard core off roaders rely on the stock radiator loop to cool their automatics. Additionally if significant water gets into the fluid it can cause the tranny to regurgitate the fluid; both of these also apply to power steering units.
Speaking of additional cooling an oil cooler is not out of place on a crawler or heavily loaded vehicle, IMO.
Probably the biggest disadvantage to single pump automatics (a very few automatics were designed with both a front and rear pump) is their inability to use engine braking on hills putting it all on the brake system.

I often hear complaints about having to shift a manual too much off road; the transfer case in low range usually drops the gearing by a factor of 2 or more so most vehicle can easily start and drive in most gears, some shifting is occasionally necessary for terrain/obstacle changes.
For me, I have found that a 4:1 first geared ~.7 overdrive 5 speed to be the best fit. The 6 speed that I considered only split one of the 5 speed gears for no significant advantage on or off road as far as I could see.
The CJ with 35s averages 18 to slightly better than 22 MPG, terrain dependent, on the interstate; with mileage mods to the 4.0, the modified & injected 258 engine that I used to have would do better (had a broader and higher torque curve).

Driver habits/experience (and expectations; realistic or not) as mentioned is probably the most important factor in how well a vehicle works on and off road, IMO.

Enjoy!

 

[kojackJKU]

I have 3.73s now in my JKU sport auto. its a 2011 with 33's. I am going to put 4.10s in it, just for a little extra on the highways. I don't need lower crawl ratios etc.

 

[jeep-N-montero]

Insert dead horse smiley....