Originally Posted by hpjaz
For daily use, is the 4.10 a "must" upgrade for the 6 speed manual?
Or, does the 3.21 get the job done around town?
I've got the 6 speed and the 3.21 axle and was worried about how the vehicle would respond with 33's on it. I figured I'd want to upgrade to the 4.10s. After driving the vehicle for two months around town, on the freeway, and in snow, ice, and dirt - I'm sticking with the 3.21s.
For starters, the 6 speed has a pretty low first gear. It's low enough that, unless I'm on a hill, I don't give it throttle when letting out the clutch from a stop. And, the gears are grouped relatively closely – allowing you to keep RPMs just where you want ‘em.
With the stock tires I was turning 2300 RPMs at 70 MPH in 6th. With the 33's I'm at 2400 RPMs at 70 in 5th - which is the gear of choice now for freeway cruising. (note: all figures corrected for tire change) It'll still pull 6th with the 33s, but at speeds of around 80 MPH+ to keep up the RPMs - I think it's a bit much for safety and for legal reasons.
As I've opined in other threads, I think the JK has an easier time at speed due to improved aerodynamics. My 4.0 TJ, which has the 4.10s and 33's, strains to keep up with traffic above 70 MPH and pretty much hits a wall about 80 MPH. The 3.8 JK with 3.21s and 33's I thought would do worse on the freeway - but it actually does better. I don't know what the top speed of my JK is, but I can tell you it's not 85 MPH - because when I was at that speed on the freeway last week I still had power left to accelerate.
Now, admittedly, my JK has an advantage over my TJ that's pretty obvious - the TJ is an auto and the JK is a stick. The ability to match the best gear to engine speed and conditions always goes to the 6-speed and not the auto. However, given that, I would think the TJ's higher axle ratio of 4.10 would more than compensate in performance. It doesn't. Both vehicles have 33's and the JK is quicker, faster, quieter, and more fuel efficient (TJ = 15 MPG, JK = 18 MPG).
As I continue to say - I wonder if people who are commenting that the 3.8 V6 is a dog (compared to the 4.0 I6) have actually spent much time driving both. If the 3.8 is lacking something in comparison - I've yet to see it. Quite the contrary, I've found the 3.8 to be more efficient and effective in every day use.