So long story short, when I got my JKU the mileage was phenomenal. I could put up 19.5 MPG in a mix of city/highway driving and it was surprising to say the least. VERY awesome mileage all things considered. Put 31s on, dropped to 17.5. Put 33s on and dropped to 15.5. I'm starting to want gears now.

Been reading and learned that 4.56s would put me at 2100RPM @ 70. 4.88s would put me at 2350ish @ 70. Light bulb came on in my head.. "If I run through a few tanks with the overdrive off and keep the engine cruising at 2,300RPM... I'll be able to see whether that would truly help my mileage before sinking money into gears."

So, this week I've been driving with no overdrive. My normal routine is a 4-lane 55 MPH highway to work. I'm turning 2,300ish RPMs with the cruise set.

So far, my mileage is horrendous. I'm seeing 13.5 MPGs driving in this RPM range. If I'm getting mileage this bad at 55, what would it be like at 70?? Gears aren't looking quite as nice right now.

One thing I'm noticing though....with the overdrive turned OFF, the trans. does not seem to ever lock the torque converter. You can watch the RPMs fluctuate even though the speed doesn't. Different times of the day result in different RPMs on the same road. Different weather conditions do too. This seems to be showing that the converter does not lock in 3rd gear. That could be a big reason the mileage is so horrible.

With gears, in theory the convertor would still lock in overdrive and so your mileage could still be better than what I'm seeing. Hmm.. will keep studying this...

 

you're on the right track - I put 35's and 4.88's in my autp which equate taller than stock, around town rpm's are in check so mileage is great, on freeway at 70+ it varies from 15 - 21. depends on headwinds, and terrain.

mostly flat, 21 mpg - long trips where the speed can stay steady using cruise control I avg. 19.5... I'm changing up to 37's, should get me closer to stock 4.10 ratio, we'll see what happens.

I like to lock out OD when towing trailer and right now, If I go 80 with OD on I'm close to 2700rpm, OD off and 3800 rpm. mileage still varies with lift.

 

Even if you could easily change the gearing with each tire size change, the tires themselves probably give you worse mileage.

• Wider tires have more rolling resistance than narrow tires.
• Off-road tread has more rolling resistance than street tread.
• Bigger tires raise the Jeep causing more wind resistance.

You are not likely to get back to 19.5 without the same tread width, tread pattern, and ride height as stock.

No free lunch.

 

true - I just want to be able to lock out OD and not have rpm's hit max torque and flatten out so quickly, not so much about mileage.

I did a trip through Colorado I-70 mountain pass and while I was able to hold 65 all the way up, on the steepest part, power/torque flattened out and I redlined but system took over and stopped me from damaging anything. Gotta say that's a pretty damn strong engine / trans for the stress I put it through...

All else, I like the 4.88's, 35 combo, power is good mileage is decent when wind is not blowing 30mph LOL ...

Going to try 37's for testing - see if I can find some used or rent them?

Next year Hemi - I miss the Hemi power curve in the GC i had

 

Yeah I'm not looking for some magical boost to get me 20 MPGs. I just want to get something back. If I could get back into the 16.5 to 17 range, I'd be very happy. Every 1 MPG is worth $30 bucks per month.

With 35s, it would be a no-brainer. I'd need to regear. But with the 33s I'm on that border line. The Jeep still drives OK. It's got less power and braking obviously, but it's liveable. At the same time though, it does downshift a LOT now and that kills mileage. I could drive it like this and be happy with 15.5 MPGs. Or, I could regear and hope to get a little mileage and power back. I dunno... it's a tough call.

I'm mainly just using this OD off experiment to try and guage how the JK reacts to turning 2,300 RPM on the highway, and so far it's not liking it at all. But...like I said, are there other factors coming into play here? OD OFF only simulates having gears once you are at cruising speed. It doesn't simulate how gears help when taking off from a stoplight. Also, OD off seems to keep the converter unlocked which is a big no-no if you want MPGs.

 

Hey, quick question about how you are getting your mileage counts... Are u getting it from the dash display or are u calculating it by hand (mileage/gals filled at pump)? If its from the in dash display that number may be off due to the computer wanting to see smaller tires. Soo your mileage may not be as low as you think. Let us know! Later!

 

mileage hand calculated - not the dash guage

gears and tires reprogrammed and checked with GPS

 

Yup, I calculate by hand, log it into fuelly.com and also an Excel spreadsheet, and then compare to dash display (which is consistently 0.9 MPG too high).

I'm anal about mileage and I have experimented with various things on multiple Chrysler 4x4s both gas and diesel in recent years. I'm learning a lot about these critters though...and what makes them more effiicient and what doesn't.

Truthfully, the least expensive way to get my mileage back up would be to trade my 33s for some 32s because then I could go back to a 6-ply tire and shed a ton of weight, and I'd only be giving up 1/2" of ground clearance. That would be a lot cheaper than regearing.

But I'm keeping my options open while I learn this thing. I've only had it for 6 months.

 

Just let me know if you want any explanations as to how everything works for possible insight on how to get it to run most efficiently. I am an engineer with a masters in fuel science (specifically applied to I.C. engine combustion), I have worked on experimental vehicles, and I have worked in GM's powertrain division on the 4.5L light-duty diesel that unfortunately was axed when GM almost went under. I have chimed in on threads like this in the past but my opinions are not always well-received because I don't BS anyone and most of the time what supposedly works for one person on this forum does not work for others. It is very driver dependent as to how you can best maximize your fuel economy and if you honestly characterize your driving style for me then I can also help give insight as to whether re-gearing will increase or decrease your overall fuel economy.

As a brief overview for people with an automatic transmission: If you live in a flat or slightly hilly area and most of your driving is highway driving, re-gearing will almost certainly HURT your fuel economy (engine spins faster in OD). If you live in steeply hilly or mountainous terrain and most of your driving is speed limit (65 - 70 mph) driving with not a lot of passing (NOT AGGRESSIVE) then re-gearing will most likely HELP your highway fuel economy (you will need to down-shift on hills and mountain passes much less). If you live in steeply hilly terrain and most of your driving is on the highway and if you ARE an aggressive driver, then re-gearing will HURT your overall fuel economy (constant passing and fast up-hill driving will still cause your Jeep to downshift just as much as before re-gearing and now your engine will be spinning even faster after downshifting). If most of your driving is around town there will not be very much change in your fuel economy as the fuel used to accelerate between traffic lights and stop signs is the vast majority of your fuel burned (so if most of your driving is city driving the re-gearing will never make enough of a difference to pay for itself).

 

Well, I live in Kentucky, so clearly the landscape here is anything but flat. We've got a lot of hills and some mountains. Every trip whether it be .5 miles or 50 miles is going to see uphill and downhill travel.

I used to have a spirited driving style. But when gas hit astronomical levels in 2008 I really started to back away from that. Today, my driving is VERY conservative. Slow acceleration, slow deceleration, trying to anticipate red lights, trying to anticipate what traffic will do, staying at the speed limit always, etc. I really do try to milk good mileage from anything I drive.

In my studies with prior 4x4s, cold air intakes didn't do a damn thing for me except make more noise. Exhaust made only a very slight improvement, but I lost low end torque and the tradeoff wasn't worth it. Going to electric fans, etc. to shed engine load did help me...but the JK already has an e-fan. Gearing made a HUGE different on my Jeep YJ...but not as much of a difference on my Jeep ZJ. I've tried a lot of different things. But it seems to really come down to weight, and engine load.

I still would like to know why the torque converter will not lock up in 3rd gear on these JKs. That is not good for mileage OR the transmission. Very odd...