Is 33s on a stock 2006 TJ with 3.73 gears asking for trouble?

Rob5589 said:
1900 is lugging the engine, not too mention 1200-1300. Hell, idle is about 800.

A couple things come into play. You have to get beyond "getting it back to stock rpm's." It is no longer stock with 33/35 inch tires. They are much heavier, wider, and need more than "back to stock" gearing to keep forward momentum. Plus you are going to be lifted, so you make the "block" we drive even worse.

The 4.0 can run at 2600-2800 all day without a sweat and do very well efficiency wise. Why did Jeep run such high ratios (3.07, are you freakin kidding??) is partially to meet EPA requirements. Not necessarily mpg, likely more for emissions. More rpm's generally mean more emission output. That and many people think 4.11 is low. Mention deep 4.XX gears and many question you; say 5.XX and they think you are nuts. We used to build a bunch of Toyota 8" back in the day and 5.29 to 5.71 were the recommendation. Those guys loved 36" Swampers/Cepeks on the little 22r/5 spd/8" diffs.

To add: scrap those "charts" that are floating around. Again, they are strictly mathematical calculations to get back to stock rpms and in whatever is the 1:1 gear, overdrive is not taken into account.
Click to expand...

Well with larger tires the math makes perfect sense. I have stock 28" tires on right now and will be going to 30" tires soon. I imagine that my 3.73's would only be a little outsized by the 30" tires? Or maybe they are just perfect?


Sent from my iPhone using Tapatalk
 
JP98 said:
@LCL-Dead You may want to try driving with your O/D off. See how that does for you. And I'm assuming Semper Fi by your location.
Click to expand...

Odd note: Nashorn's user ID# is 1775. I want it. (Yes, Semper Fi) :D

Much higher with O/D off, of course. Chugging around on base, when the OD kicks on in 4th gear, I scoot along at about 1500rpm @ 45mph. Given some of the explanations above I'm tempted to just turn the O/D off until I'm off base and can legally hit speeds above 50mph.

Here's my RPMs at speed.

xAz2u4O.jpg

5IRQg9Y.jpg


The most surprising thing to me, of course, is that I managed to snag both of those pictures without either a check engine light or my "front lock" light blinking (Believed to be a wiring issue).
 
Nashorn101 said:
Well with larger tires the math makes perfect sense. I have stock 28" tires on right now and will be going to 30" tires soon. I imagine that my 3.73's would only be a little outsized by the 30" tires? Or maybe they are just perfect?


Sent from my iPhone using Tapatalk
Click to expand...
For your combo, I wouldn't spend money on a gear swap. The 4.0/3.73/30 will be fine. A 30x9.50 isn't all that much heavier or wider than the 28x8.50.

Say you were in my boat; 2.4/3.73/31, then a gear swap is advisable (and exactly what I did). You have to gear to the entire drivetrain, not just tire size.
 
  • Like
Reactions: StG58
LCL-Dead said:
Odd note: Nashorn's user ID# is 1775. I want it. (Yes, Semper Fi) :D

Much higher with O/D off, of course. Chugging around on base, when the OD kicks on in 4th gear, I scoot along at about 1500rpm @ 45mph. Given some of the explanations above I'm tempted to just turn the O/D off until I'm off base and can legally hit speeds above 50mph.

Here's my RPMs at speed.

View attachment 18737
View attachment 18738

The most surprising thing to me, of course, is that I managed to snag both of those pictures without either a check engine light or my "front lock" light blinking (Believed to be a wiring issue).
Click to expand...
Looking at your profile pic, I am guessing larger than stock tires. So if you have not compensated by swapping speedo gear or electronic converter box, your speedo is slow.
 
Nashorn101 said:
I've been trying to find the sweet spot for my Jeep too, and i've always thought that above 2000rpm's was too much if you want good fuel efficiency - ? I say this because the "shift assist arrow" always pops up at around 1900rpms, which leads me to believe that anything above that is not "fuel efficient" in the eyes of whoever programmed this at the factory, and the service manual clearly states that this is a fuel efficiency feature. So which one is correct?
Click to expand...

If fuel efficiency is a goal gear it for performance in direct, 3rd with the 4speed automatic, 4th with the old 5speed and 5th with the 6speed. Then you can use OD to drop RPMs when you don't need 2500rpm. Which is exactly why OD became popular 40 years ago.

Most cars today hit top speed down 2 or even 3 gears and lose a bit of top end with every upshift. Definitely if you gear for 2900rpm at 60mph in OD you will never get great gas mileage. But if you gear for 2500rpm in direct then you can drop a gear or two and you will hit 2900rpm at 60mph while still having the option of dropping rpms when cruising. Nothing wrong with 1800rpm if you don't need the power.

I have a TJR on 33s, 4.10s. I wish it had 3.73s.
 
  • Like
Reactions: Kyle_W
Rob5589 said:
Looking at your profile pic, I am guessing larger than stock tires. So if you have not compensated by swapping speedo gear or electronic converter box, your speedo is slow.
Click to expand...

29.5", actually, just a hair larger than stock and according to the GPS app I use to track trails, the speedo is accurate to within 1-2mph.

Now you've got me worrying that maybe the gearing has been changed despite the 4.10 tag on the rear diff.

ukeVgUm.jpg
 
billiebob said:
If fuel efficiency is a goal gear it for performance in direct, 3rd with the 4speed automatic, 4th with the old 5speed and 5th with the 6speed. Then you can use OD to drop RPMs when you don't need 2500rpm. Which is exactly why OD became popular 40 years ago.

Most cars today hit top speed down 2 or even 3 gears and lose a bit of top end with every upshift. Definitely if you gear for 2900rpm at 60mph in OD you will never get great gas mileage. But if you gear for 2500rpm in direct then you can drop a gear or two and you will hit 2900rpm at 60mph while still having the option of dropping rpms when cruising. Nothing wrong with 1800rpm if you don't need the power.
Click to expand...
Gearing for 2900 at 60 in OD is pretty deep. I am at 3000 at about 67 in 6th, running the 2.4 however. I don't think anyone is saying go that deep, unless a more dedicated off roader.
 
  • Like
Reactions: billiebob
LCL-Dead said:
29.5", actually, just a hair larger than stock and according to the GPS app I use to track trails, the speedo is accurate to within 1-2mph.

Now you've got me worrying that maybe the gearing has been changed despite the 4.10 tag on the rear diff.

View attachment 18739
Click to expand...
No, looks right based on the gear ratio calculator. 4.10/29.5/.69/55 mph.
 
  • Like
Reactions: LCL-Dead
Jerry Bransford said:
For 33's, I'd go no less than 4.88 though I personally would go 5.13. For 35's, no less than 5.13 though I'd go 5.38. My 4.88 gearing with my 35's absolutely sucks on the street and highway with the automatic but that's what it came with after my previous TJ was stolen.
Click to expand...

Would you run 35's with a 5.13 gear ratio?
 
billiebob said:
Nothing wrong with 1800rpm if you don't need the power.
Click to expand...

There is something wrong with 1800 rpm: lugging.

You gain absolutely nothing by running the engine that low. No, gas mileage does not improve at 1800 rpm. Going 65 in 4th I used to get better gas mileage at 2500 than I did at 1900 in 5th.
 
  • Like
Reactions: JP98, Jerry Bransford and Rob5589
Brantley said:
There is something wrong with 1800 rpm: lugging.

You gain absolutely nothing by running the engine that low. No, gas mileage does not improve at 1800 rpm. Going 65 in 4th I used to get better gas mileage at 2500 than I did at 1900 in 5th.
Click to expand...
Common misconception, many think the same way. RPM's that are too low and causing the trans to repeatedly downshift will have a higher fuel consumption than a nice constant rpm. Most of your fuel use comes from accelerating from a stop. If you have to stomp the gas to get moving, you are using more fuel than a nice and easy pedal and acceleration.
 
  • Like
Reactions: Brantley
Rob5589 said:
Common misconception, many think the same way. RPM's that are too low and causing the trans to repeatedly downshift will have a higher fuel consumption than a nice constant rpm. Most of your fuel use comes from accelerating from a stop. If you have to stomp the gas to get moving, you are using more fuel than a nice and easy pedal and acceleration.
Click to expand...

Yep. Unfortunately people continuously spread the misconception that dropping the rpm as much as humanly possible while continuing to roll down the road increases gas mileage. Mine and many others' experiences do not represent that misconception.
 
Rob5589 said:
Common misconception, many think the same way. RPM's that are too low and causing the trans to repeatedly downshift will have a higher fuel consumption than a nice constant rpm. Most of your fuel use comes from accelerating from a stop. If you have to stomp the gas to get moving, you are using more fuel than a nice and easy pedal and acceleration.
Click to expand...

Drop a gear, Then tromp the pedal.
 
Rob5589 said:
Gearing for 2900 at 60 in OD is pretty deep. I am at 3000 at about 67 in 6th, running the 2.4 however. I don't think anyone is saying go that deep, unless a more dedicated off roader.
Click to expand...
With SE.... You definitely want deeper gearing especially on the highway. I think the SE Auto came with 4.56 for STOCK tires. There is not much reserve in the 2.4L.
 
  • Like
Reactions: StG58
billiebob said:
With SE.... You definitely want deeper gearing especially on the highway. I think the SE Auto came with 4.56 for STOCK tires. There is not much reserve in the 2.4L.
Click to expand...
The 2.4 SE's had the 4.56 gears in them with the automatic for a few years.

All SE's can use all the gear you can stuff in them. And limit the tire size to 32". They still get the same gas mileage as the 4.0.
 
Top Bottom
Back