dog

I've got a 2010 3.8 rubi 2dr with 70k. I had 33s on 1.5" spacers and in the last few months I've seen a drop in MPG from about 14 around town to about 11 in town. I don't haul ass, but I have plenty of stop and go. Most speed limits in town are 35, probably max 50.

Then, like the bright young lad that I am, I sprung for new wheels and tires (37 MTR/Ks on 17 Black Rock Steel). Now (naturally, this hasn't helped...) but I'm getting 9.8mpg

From what I understand, guys with similar setups are getting 30-40% better fuel mileage than I am (at least 13 or 14 MPG). Is this reasonable? Should I worry?

I'll be switching over to synthetic tomorrow (Mobil 1), I'll check the air filter, look into running an injector cleaner, and later probably swap out plugs for iridium? I've also been told to make sure I don't have a cracked manifold... so I'll look into that. I have a ton to learn about the vehicle and was hoping for a little feedback. Am I missing anything?

Thanks for any and all opinions or leads to where I can learn a little more.

HawaiiMark

How are you calculating your MPG? If you put bigger tires on and did not adjust your speedo accordingly then the jeep is actually going further and faster than the speedo is indicating which means when you calculate your MPG is correspondingly worse.

Unlimited_X

the older your Jeep gets the worse gas mileage it will get. True for all cars but that is a significant difference. If you want better mileage, get some smaller road tires and top off the PSI to the max.

dog

You know what, I hadn't even considered that. This is how dumbasses like me learn, though. That probably accounts for some of the mpgs... I'll fill up today and try and get a tank/mileage count that's a little more true over the next couple weeks.

So I had 33's, and went to 37s, and the vehicle was stock 32s new. I suppose it's possible it was never reprogrammed from the 32s... which means I'm still calculating on a ~100" circumference instead of the newer ~116" circumference. That's a 16% increase in circumference. That's 5462 revolutions per mile instead of 6336 revs/mi. I'm getting 86% of the original revs per mile. So if I'm only showing 9.8MPG but that's 86% of my "real" mpgs, that means my "real" mpgs are 11.39.

I'm not fantastic at math, so I probably screwed that up. So 11.5ish mpgs isn't as bad as I thought. Considering I'm running 4.10 gearing... I guess all-in-all I don't really have anything I should be concerned about? That's still less than I would expect having read other people claiming 14-15, but maybe I'm just paranoid.

In any case, gearing and some front axle armor/c gussets and bjs are next. Would 5.38/5.86 gearing help or hurt?

dog

I'm not concerned about getting shitty gas mileage in principle (inb4 "get a prius"), I'm just wanting to make sure I don't have such poor mileage that I should be looking for a greater issue somewhere!

Joe_B

You have a rubi so you can go 5.38s

I run 5.13 on 37s in a JKU and it does the job but deep down I want 5.38s. 4.10s with a 3.8 auto and 37s is really stressing your drivetrain. Regearing is the best thing you can do for your jeep. I don't understand why people don't do it sooner.

And reset your speedo and you'll see an improvement in your mileage.

Good luck!

dog

Yeah according to the infamous gear chart I should be at 5.38 or 5.86. I leaned toward 5.86 but I'm still deciding. I've only had the new wheels/tires for a week, and the regear is the very next thing I have planned. Thanks for the heads up!

Joe_B

You can't go any lower than 5.38s in a jk dana 44.

nthinuf

Yep, 5.38's is the highest (numeric) you can go. Your best bet would just be to read through a few of the thousands of "what gears should I get" threads that are already here. Plenty of opinion to go around. Just make sure to note the setup of whoever is replying, since 3.6's and 3.8 6-spds have different gearing needs. Also look at where they live compared to you, high altitude with lots of hills/mountains or sea level with totally flat roads. Or all in town/offroad compared to a long daily commute at 80+ on the freeway. It can all make a difference in the replies you get...

For me, on my 3.8 4door auto, I have always liked having higher rpm's. 5.13's with 33's, then 35's. Then went to 5.38's with the 35's, and finally got 37's. Mainly at altitude in Colorado, but spent a lot of time near sea level in Texas. So my opinion is ... don't fear the gear...

And definitely get a programmer/tuner. Aev Procal or Superchips FlashCal will set the new tire size, and gears when you get them done. You should notice a decent difference in the shift points from that. And if you step up to a tuner, you should see 8-10 hp for normal driving. (some don't feel it seat-of-the-pants, others say it is a great improvement)

Also, you should be pressing the OD/Off button as soon as you turn the ignition.

Maertz

You could go 5.38 in a d30 also if your feeling brave enough. Rubi just changes strength of pinion/ring gear.