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06' TJ Highway Driving

12K views 25 replies 14 participants last post by  Garry125 
#1 ·
Hi all,
I've had some great help with the folks on the forum before and figured that I might ask for some simpler help this time.
I just started college this week and due to a limited budget am commuting the 40 miles there and back every day. Being new to driving a manual, I haven't had the time to gain good knowledge of interstate driving yet, and being the only one in the family with a manual, I have no one else to ask. I live in a very mountainous area and commuting over the past few days I have noticed that I have a lot of trouble keeping a decent speed, especially climbing the steep hills along the interstate. I have no interest in becoming a speed demon, but I'd prefer not to drop below 65 on the interstate which I find myself doing on almost every hill. My original understanding when I was taught to drive a manual was to always use the highest gear that fit the speed, with the lowest rpm achieved the most efficient option when maintaining speed.
I have an 06' TJ with a 6 speed manual transmission. Mostly stock, as far as I know, with 29 inch tires. With that in mind, my knowledge was that ideal rpms for inclines or anything requiring power was between 2k-3k, which my jeep will maintain for the most part on those inclines in 6th, but I still lose power climbing the hill. When I do shift down to 5th I see no increase in power and my rpms will go up to around 3k. This makes my jeep sound like it's working harder, but I see either no power increase, or, I actually start slowing down.
Is my lack of power climbing these hills operator error(most likely)? Or is it actually a power issue with my jeep? If it actually ends up being the latter, what do I do about it? Re-gear? Cold air intake? Turbocharger? Accept it? I have no idea since I have no experience with the mechanics of almost any vehicle. I'm ready and willing to dig down and start working on any part of my jeep, I just don't know what to work on and need instruction.

Thanks in advance for the advice!
 
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#2 ·
I have a 2005 with the 6 speed, 3.07 gears, and 31" tires. It is definitely underpowered with those gears and any slight hill I hit will drop my speed like a brick in 6th. If I downshift into 5th I'm usually ok. You probably have 3.07 gears as well and they aren't very well liked. A re-gear would probably help but that's a big expense and I don't think that is your solution. I feel like you should be able to maintain speed even up a hill in 5th gear, so maybe you are losing power somewhere in the engine. I will let some more experienced people chime in about that.
 
#3 ·
What size are your tires (as listed on the sidewall)?

When you are on relatively level ground and traveling at 70 on the interstate, what is the indicated RPM?

It would also help if you would put your general area in your profile, that way we would know approximately how severe the grades are and what the altitude is. For example, if you are traveling north on I-77 between the VA/NC line ant the I-77/I-81 interchange, you would be hard pressed to maintain 65 MPH a TJ with 3.07 gearing.

The six speed has a rather mile OD in 6th (.83 I believe) and is 1.00 to 1 in 5th. The NV3550 5 speed it replace is .78 in 5th and 1.00 to 1 in 4th (it's what I have in my Rubicon).

The 3.07 gear set, if that's what you have, is intended for economy, not performance, so you might not be able to maintain 65. If you can find a shop that has an in vehicle dynamometer,, and find out what they charge to run your vehicle. Basically you park with the rear wheels on rollers, plug in the equipment to the vehicle and run it through the tests. It will tell you what your true power is vs. stock as shipped.

I know the 4.0L is rated at 190HP but don't remember the torque.
 
#4 ·
My opinion is if your on a budget & have a long daily commute, A jeep is one of the worst possible vehicles to own for that situation. Plus you have to deal with hills, No thank you. Get rid of it & get a commuter car then buy a jeep later down the road when you are settled financially and can make it a toy or whatever.
 
#15 ·
I can understand where you are coming from, but I like having a small 4x4 vehicle, especially when I get 4 feet of snow in a 130ft long driveway in the winter time. I love my jeep, and I spent months looking for one I could afford. I have no plans on getting rid of it any time soon.
 
#5 ·
Jeeps do best in the 3000+ rpm range on the highway, especially on hills. Don't be afraid to rev the engine. It won't hurt a thing. If you have 3.07 gears, you get swap in some 4 cylinder axles for around $500 or less a set (front and rear). A regear where I live runs around $1500.
 
#6 ·
Maybe you will have better luck than me but I have been searching for those axles for a month without even a hint of anything on Craigslist and the local Jeep junkyard wants almost $2k for both axles! Hahaha!
 
#7 ·
You seem to be reluctant to use your gears. Nothing wrong with downshifting to 4th or 3rd to keep your speed up when you have to. It won't hurt anything. You engine doesn't redline until 5200 rpm. Certainly nothing wrong with running it up over 4,000 RPM for a few minutes when needed. Probably easier on the engine and transmission than lugging it.
 
#8 ·
Keep your rpms up. There are several long highway climbs out here where I am in 3rd pushing 4k rpm in order to maintain 55mph. ~2500rpm is a good cruising rpm. If I'm pointed downhill, then I will often upshift. Downshift before the hill begins, watch traffic ahead and do whatever you need to do (within reason and safety) to keep your speed while climbing.
 
#10 ·
This may be of little help, but for comparisons sake I'll give my experience anyway.

When I picked up my latest TJU (after taking a long hiatus from driving 4.0Ls) I was amazed at the amount of on demand torque I had, geared with 3.73s and stock 30" tires. It was hitting right about 3000 RPM at 70 MPH on the highway which was perfect for overall driveability. Now the Jeep is geared to 4.88 with 37s and is almost exactly the same...I notice virtually no difference in the driveabiltiy, and is also turning 3000 RPMS right around 72 MPH.

Lesson of the day...proper gearing is important! :)
 
#11 ·
Stock tires 205/75/R15 (27"): With 3.07 gearing
If you had stock 27" tires and put 29" tires on it your gear calculation would be= 2.86
If you had stock 27" tires and put 28" tires on it your gear calculation would be= 2.96
If you had stock 27" tires and put 27" tires on it your gear calculation would be= 3.07
If you had stock 27" tires and put 26" tires on it your gear calculation would be= 3.19
If you had stock 27" tires and put 25" tires on it your gear calculation would be= 3.32

The numbers seem small BUT (for instance)
If my Jeep came with 27" tires and 4.10 gears stock.
And I put 36" tires on it my gear calculation would be= 3.08

Jeeps that come stock with 4.10 gears and put 35" tires on it (calculation)= 3.16
Jeeps that come stock with 4.10 gears and put 33" tires on it (calculation)= 3.35

You can see how much tire size affects your gearing..........
People who lift their Jeeps with 4.10 gears and put big tires on them, are geared in a ratio as stated above. (Pretty close to yours at the end of the game)

The math does not lie.
That's why they sell Jeeps with 27" tires and 4.10 gears for above standard torque and off road capabilities but don't have good highway speed @ 2500 rpm.

Then sell Jeeps with 27" tires and 3.07 gears for standard torque and off road capabilities and have good highway speed @ 2500 rpm.
 
#12 ·
The math does not lie.
That's why they sell Jeeps with 27" tires and 4.10 gears for above standard torque and off road capabilities but don't have good highway speed @ 2500 rpm.
I think of this math is correct to a degree, however you are still forgetting two extremely important factors; engine and transmission. There's a reason TJ SEs have 4.10 gearing despite being on 27"-28" tires, as there is also a reason that they recommend 4.88 gearing when you have the 42rl transmission, 33" tires, and the 4.0 compared to running 4.56 gearing when running the NSG370 or NV3550 transmission in the same situation with engine and tire size.
 
#22 ·
According to my calculations, if you are going about 70 at 2800 RPM, your axle ratio would have to be 4.11. That makes sense to me, for at 70 in my Rubicon with slightly larger tires I turning 2550. Now according to my 2004 brochure, the 4.11 differential gears were not an option on any Wrangler. They were standard on the Rubicon and the 2.4L engine. But, you have the 4.0L engine and it’s not a Rubicon (locking diffs, 16” wheels on Dana 44s F&R). However, if you are going 70 at 3150 in 5th gear (1.0 to 1, or as we sometime call it, straight through since there is no reduction or increase from the engine to the transfer case. That then give us 3.73 as a diff ratio.

Here is the calculation: RPM/Final Drive Ratio/Diff Ratio*60*Rolling Radius*2*3.1416/12/5280=MPH.

The Rpm is 3150 in this case, the final drive ratio is 1, the differential ratio we substitute for, 60 is a constant to convert minutes to hours, the rolling radius needs to be calculated, but in this case I used 13.94”, 2 is a constant to convert radius to diameter, 3.1416 is a constant (Pi) to get the rolling circumference, 12 is a constant to convert inches to feet and 5280 is a constant to convert feet to miles.

If you want to work it backwards, MPH*5280*12/3.1416/2/13.94/60*Final Drive Ratio*3.73=3148 (RPM in 5th at 70 MPH) and 2520 if you substitute .8 for the FDR instead of 1 (6th at 70 MPH)

Logic for constants – 60 takes us from rev per minute to rev per hour, we need the rolling circumference of the tire which then converts revolutions to inches per hour, the 12 converts inches per hour to feet per hour and the 5280 converts feet per hour to miles per hour.

Rolling radius – you have 235/75R15 tires. The 235 is the width of the tire in mm, so convert that to inches (divide by 25.4 mm/inch) and we get 9.25 inches wide. The height of the tire is 75% of the width, in this case 6.94 inches (height from tread to mounting surface of the wheel). We need ½ of the wheel diameter, it is a 15” wheel so the radius of the wheel is 7.5”. Add this to the height of the tire and you get 14.44” or a 28.8” tire (unloaded). The rolling radius is less than the unloaded radius by about a ½ inch (the squish factor of tires). While the actual may be more or less, it is a good estimate. The actual can be measured when on level hard pavement (like a parking lot) measuring from the ground to the center of the wheel. The pavement does not have to be level, just flat.

I have an Excel spreadsheet (used to be in Lotus 1-2-3) and any time I do a tire calculation it goes in the spreadsheet. It started when I was doing the calculation of what effect I was getting after putting LT235/75R15 tires on my new ’91 YJ which came with LT225/75R15s. All I had to do for yours was substitute the RPM, the final drive ration and I would get a result.

Initially I had 3.07 but I think you would have noticed if you were going 95MPH in 6th at 2800 RPM, so then I used 3.73 but that gave 78 MPH. Those were the only diff ratios available in the 2004 brochure for a Wrangler with the 4.0 and was not a Rubicon. It is more likely you were actually at slightly over 2500 in 6th and have 3.73 diff gears. Run the original build sheet and see what it says.

Some time you may want to do a check on the speedometer as well. But I don’t think it is off too much. If you are travelling at 70 in a 70 zone, you would be passing a few people and getting passed by a few people. If you were going 78 you would be passing almost everyone and getting passed only rarely. (At least down here on I75).

You can also verify the differential ratio by jacking up one rear wheel (while on level ground and chocking front wheels. Put the valve stem in a definite location (ie: at the top or bottom), put a mark on the driveshaft (tape, chalk, etc) and then slowly rotate the tire and count the drive shaft revolutions. If it is over three and almost 4, then you have 3.73, if it is slightly over 4 then you have 4.11, if it is slightly over 3, then you have 3.07. IMHO, the latter two are unlikely.

Sorry to be so verbose, but I wanted to explain fully.
 
#25 ·
OP stated that even if 5th (1.00 to 1) and over 3,000 RPM he can't maintain speed. If he drops it into 4th (1.25 to 1) then he will be turning almost 4,000 to maintain 70. That would be one heck of grade.

In an article on grades on Interstate highways, it mentioned that western Interstates generally have a grade of 6% or less, while in the East they are 8-9% with a few at 10%. The offset is the Eastern grades are shorter. The one that I like in good weather and will avoid in bad weather is I-81 on Fancy Gap Mountain, basically from MP3 to MP8. It is 8-9% with a vista view to the East if you are going North because the road is going up the side of the mountain. (People around the Rockies would probably call it a hill since the elevation change is only about 3,000 feet, but they are much older than the Rockies as well).
 
#26 ·
I just regeared mine after the d44 swap. It was 3.07 with 32's. Now I have 3.73. In my opinion I love the 3.73. Alot of low end power. If seriously off reading id do 4.10s but I don't do it to often. Even with stock tires I hated the 3.07 with the 32s I was constantly down shifting. It's all personal preference. If you have someone with a different ratio try a test drive. My Jeep is 06 six speed with 4.0.
 
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