Jump to content

LOW FINAL DRIVE RATIO


Recommended Posts

Hi,

 

I was wondering if anybody see's a problem with switching my 91, 4.0ho, 5spd, 4x4 Comanche to a 4.10 final drive ratio. I want to do this because i'd like to be able to tow 5000lbs. I've already towed with it currently and the 3.07 gears are just to high to tow with. It struggles at low rpm and makes the 4th and 5th gear almost useless unless you get going really fast.

 

I've made a spreadsheet using the transmission ratios, final drive ratio's, and tire size as inputs to determine engine rpm at any speed. I also calculated the rear wheel torque with several different final drive ratios using an input engine torque of 200ft-lbs neglecting driveline resistance.

 

Results at 65mph in 5th gear

Final Drive Ratio Engine (RPM) Rear Wheel Torque

3.08 1898 489ft-lbs

3.55 2188 564ft-lbs

4.10 2527 651ft-lbs

4.56 2811 724ft-lbs

4.88 3008 775ft-lbs

 

Results at 10mph in 1st gear (assume 175lbs engine torque)

Final Drive Ratio Engine (RPM) Rear Wheel Torque

3.08 2124 2064ft-lbs

3.55 2448 2379ft-lbs

4.10 2827 2748ft-lbs

4.56 3145 3056ft-lbs

4.88 3366 3271ft-lbs

 

 

Based on these results I think that 4.10 would be reasonable for highway towing. But if anybody has any insight or experience with changing their final drive ratio for this reason please let me know what you think.

 

Thanks,

-Ken

Link to comment
Share on other sites

First of all............I don't think I would be pulling a 5000# trailer with a 3500# truck :hmm:

 

Your having problems getting up and going............what about stopping???

 

The general rule of thumb is to tow with something heaver that what your towing.

 

Second..........You can upgrade to a 8.25 rear that would be a better choice than the D35, and might even be able to find one with 4.10 gears from a XJ with the 2.5 and auto tranny in it. That would be a closer swap for the D35, just burn on some new spring perches (SUA)

 

Then you would need to change out your D30 with the same gear ratio, and change your speedo gear, if you want to keep the speedo working correctly.

 

Third.........I think we're neighbors :doh:

Link to comment
Share on other sites

First of all............I don't think I would be pulling a 5000# trailer with a 3500# truck :hmm:

 

Your having problems getting up and going............what about stopping???

 

The general rule of thumb is to tow with something heaver that what your towing.

 

Second..........You can upgrade to a 8.25 rear that would be a better choice than the D35, and might even be able to find one with 4.10 gears from a XJ with the 2.5 and auto tranny in it. That would be a closer swap for the D35, just burn on some new spring perches (SUA)

 

Then you would need to change out your D30 with the same gear ratio, and change your speedo gear, if you want to keep the speedo working correctly.

 

Third.........I think we're neighbors :doh:

 

 

Well I have working trailer brakes and I also have a friend thats going to sell me a Chrysler 8.25 to use.

I was pretty sure that the Jeeps are good to pull 5000lbs from the factory. They usually come with 3.55 or 3.73 with a towing package I think.

 

Also when you pull 5000lbs don't forget there is a tongue weight that gets transfered onto the rear axle of the vehicle.

 

Real over the road Truck Drivers also have braking issues. How do they deal with it? They start slowing down alot sooner than cars and they usually don't follow as closely.

 

I agree with you though a bigger truck would be better but I don't have the money to buy one and I don't want to sell my comanche.

 

Thanks,

-Ken :chillin:

Link to comment
Share on other sites

This is Quoted from the '89 FSM -

 

Class III Hitch - 70 Series Vehicles

 

The following basic equipment is required for class III (heavy duty) trailer towing applications (IE weight=distributing /equalizer type hitch with a trailer having a maximum gross weight of 5,000 lbs/2250 kg and a maximum tongue weight of 750 lbs./332 kg)

*P205/75R15 or larger tires

*Full size spare tire.

*low profile mirrors.

*trailer sway control.

*trailer wire harness.

*Heavy duty flasher.

*heavy duty axle (with synthetic lubricant)

*heavy duty cooling system.

*heavy duty alternator/battery.

*auxiliary automatic transmission fluid cooler, and

*4.0L engine.

 

So, your right in assuming that the MJ can handle a 5000# trailer, but, are we talking about a large, wide trailer, kind of like a 6 foot X 16 foot trailer??? That's the size trailer I have, rated at 7000#, with brakes, and I've only pulled it behind a E250 and a E350 cuber. And loaded, it's got some heft behind it.

 

I wouldn't pull a trailer over 3K with out brakes.

 

I under stand you don't have a choice, but then again, your figuring final drive at 65 mph, and with a trailer that size (5000#) I don't think I'd be hitting the thruway with it :roll:

 

Good luck.

Link to comment
Share on other sites

This is Quoted from the '89 FSM -

 

Class III Hitch - 70 Series Vehicles

 

The following basic equipment is required for class III (heavy duty) trailer towing applications (IE weight=distributing /equalizer type hitch with a trailer having a maximum gross weight of 5,000 lbs/2250 kg and a maximum tongue weight of 750 lbs./332 kg)

*P205/75R15 or larger tires

*Full size spare tire.

*low profile mirrors.

*trailer sway control.

*trailer wire harness.

*Heavy duty flasher.

*heavy duty axle (with synthetic lubricant)

*heavy duty cooling system.

*heavy duty alternator/battery.

*auxiliary automatic transmission fluid cooler, and

*4.0L engine.

 

So, your right in assuming that the MJ can handle a 5000# trailer, but, are we talking about a large, wide trailer, kind of like a 6 foot X 16 foot trailer??? That's the size trailer I have, rated at 7000#, with brakes, and I've only pulled it behind a E250 and a E350 cuber. And loaded, it's got some heft behind it.

 

I wouldn't pull a trailer over 3K with out brakes.

 

I under stand you don't have a choice, but then again, your figuring final drive at 65 mph, and with a trailer that size (5000#) I don't think I'd be hitting the thruway with it :roll:

 

Good luck.

 

Ya your right, its to bad there arent more J10's or J20's around in halfway decent condition.

Link to comment
Share on other sites

I would have to agree with the OEM brakes being more than lacking when towing something that heavy. I am also biased since I tow with a PSD. :D

 

The first mod I did to my MJ (before driving it) was replace the stock set up with a 2003 WJ booster/master and an XJ prop valve. (I also added 1/2 ton front brakes to it). I can lock all 4 wheels now on 36s with ease if need be.

 

I would suggest this cheap and easy mod to you before attempting a load like that (and also to anyone else wanting to stop well). I don't know if it will actually be able to handle 5K but I know the stock brakes suck horrible.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...