I have an auto and 33's and I drive with OD off 100% of the time (3.73 gears). I'v never seen the RPM's wavering when I have cruise control on, so I think the converter is locking. I'v driven 3 hours holding 2900 RPM's consistantly and I think I would have overheated if the converter was unlocked. I pulled about 16.5 MPG during that drive and averaged 75-80 mph.

I'v been debating wether or not to go to 4.88's or 5.13's at the start of next month, but I think I'm going to try 5.13's. The RPM's will still be less then what I am used to now so I don't think I could possibly lose MPG while cruising. I should be right around 2600 RPM's which I think will be a good cruising RPM. It would be on par with 5.38's and 35's.

 

The torque converter lock-up should be something you should be able to influence with an aftermarket programmer (or you could even see whether the dealer would be willing to re-flash your JK to give it the most up to date programming from Chrysler that might have the converter lock up sooner). In my old ZJ it got real annoying because the stock ECM died shortly before I traded it for my JK and the replacement ECM was really sporadic on when it would lock up the torque converter and when it wouldn't (even in OD) so I understand your frustration with the converter not locking up when you feel like it should.

Also for the things like cold air intake and such you are pretty much right. They only add power at the high end of the engine RPM range (which is the exact wrong area you want to be for optimum fuel economy ). The only reasons I have my aftermarket exhaust are for the sound, for the weight reduction (it cut about 10 pounds off the weight by getting rid of the huge stock muffler), and for the clearance (it has already saved my JK from damage during a slow speed rear-end collision from a little car that just slide under me and propped my Jeep up on it's hood by my rear tow hook ).

Aerodynamic improvements (lower to the ground, less frontal area, lower coefficient of drag, etc.) improve highway fuel economy while weight reductions improve city fuel economy (and highway fuel economy on steep hills and mountain passes)

 

the transmission convertor clutch should be applied no later than 3rd gear partial apply can occur in 2nd

 

Dronac.. let me know how the gears turn out for you. Would like to hear how that affects your mileage using overdrive again with the gears.


Good stuff.. keep it comin'.

 

Will do. It will be about a month before they are in though. Send me a PM so I have it in my inbox to remind me to get in touch with you.

 

OK, so I'll chime in with my experience for what it's worth. I've got a 08 JKU 6-spd 3.73's with 33" tires. I have a surface street (stop and go) commute of nearly 20 miles each way with one large hill/small mountain climb and I'm seeing between 16 and 17 MPG average with closer to 18-19 highway. I know having the manual helps a lot, but I accelerate from stops pretty quickly. I know this results in poor fuel economy, however I'm only getting that poor economy while accelerating for a short time to get up to my cruising speed. I can then keep the cruising speed with better economy for a longer time then. When my friend drives it around town he drives much more conservatively than I do away from a stop. He drops my average to 14MPG consistently.

I've checked this on the dash and by hand and find the same difference. I noticed this before when I was driving a couple other vehicles (Cadillac Allante, Scoin xB, Z4) with an economy computer. when I accelerate faster for a shorter period I personally get better mileage then when I gradually accelerate. Again, this is just my observations from my experiences with my JK and some previous vehicles.

 

WXman,

The torque converter certainly is supposed to lock in 3rd, once the transmission temperature reaches ≈80°F. Mine does it all the time.

If yours isn't, you have something wrong.