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Old 02-25-2016, 09:56 PM
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Help! Terrible MPG! How to find the cause...

Not expecting my '08 JKU Rubicon to compete with a Prius in MPG, but I'm currently only getting 8-9 mpg around town. I'm running a 2.5" MetalCloak lift with Fuel wheels and 325/18 BFG KO2 tires, full front and rear aftermarket bumpers, 80k miles, etc. Sure, it's heavy and its a Jeep, but from what I've been reading, I should expect quite a bit better, no? Heck, I'm not even beating on it around town, just ordinary driving, maybe a little aggressive on the go-pedal, but nothing excessive at all. I bought the Jeep with the same wheels/tires setup as it has now, only before it had spacers/budget boost kit. Nothing else has changed and its been bad in the MPG department since I bought it. Since it seems to run great, even before and after I tuned it with the FlashPaq, I'm baffled as to what could cause it. Where to start trying to figure out what's stealing my MPG's!??

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Old 02-25-2016, 10:08 PM   #2
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When was the last time the plugs and air filter were replaced? Have you ever added any fuel injector cleaner in the tank?

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Old 02-25-2016, 10:26 PM
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When was the last time the plugs and air filter were replaced? Have you ever added any fuel injector cleaner in the tank?
I've done several fuel injector treatments through the tank over the past 5k miles because that's what I always do when I buy a used car. As for the filter, it has a Rugged Ridge snorkle, and the filter looks good. As for plugs/wires, it doesn't run rough, so I figured they were likely not the cause. I'm up for changing them on the chance I'd see some improvement.
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Old 02-25-2016, 10:34 PM   #4
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Sudden decrease in MPG can often be attributed to failed O2 sensor, even before a check engine light.
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Old 02-26-2016, 08:19 AM   #5
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It didn't sound like it was a sudden decrease. What size are your tires? There should be another number in there 325/xR18. I'm not sure what gears you have, but lower gears may help your mpg, especially with the 07-11 engine. Check out this chart for more details: Gear Ratio Chart.
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Old 02-26-2016, 08:39 AM   #6
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Have you tried resetting your avg? Push the button in, and let it reset.
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Old 02-26-2016, 09:00 AM   #7
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Is your tire size configured correctly. Get a GPS speedo app and compare your speeds going down the highway. It is off then the Jeep thinks it going less distance and using a certain amount of fuel so the MPG calculation will be off by the percentage your speed is off.
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Old 02-26-2016, 09:06 AM
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It didn't sound like it was a sudden decrease. What size are your tires? There should be another number in there 325/xR18. I'm not sure what gears you have, but lower gears may help your mpg, especially with the 07-11 engine. Check out this chart for more details: Gear Ratio Chart.
Right, no sudden decrease in MPG, it's been an issue since I bought it. Tire size is 325/65-18 with stock Rubi 4.10 gears. No doubt it could benefit from some 4.88 or 5.13 gears, but I'm reading lots of other people's similar rigs are in the low teens for MPG around town, some slightly better.
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Old 02-26-2016, 09:09 AM
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Is your tire size configured correctly. Get a GPS speedo app and compare your speeds going down the highway. It is off then the Jeep thinks it going less distance and using a certain amount of fuel so the MPG calculation will be off by the percentage your speed is off.
Speedo is spot on, tuned with the FlashPaq tuner, verified with GPS. As for MPG calculations, I do it the old fashioned way, at the pump with a little math.
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Old 02-26-2016, 10:35 AM   #10
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3.8, lots of heavy accessories, heavy wheels and tires, geared low. Yup, you about right.

BTW, my '12 JKR averages 10 around town and at most 14 on the highway. I have lift, bumpers w/ tire carrier, 35" MTR/K's, skid plates, winch. All that extra weight adds up and the added rotational mass of the tires and wheels makes you drop the skinny pedal further.
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Old 02-26-2016, 12:18 PM   #11
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I've got a '08, 3.8L, 6-speed, 35s, 2.5" MC lift, front bumper, winch, full armor underneath, had 4.10s and my MPGs around town was in the 12-14 range. Highway a little better in the 14-16 range depending on conditions. I recently moved to 4.88s and don't have new MPG numbers yet, but 8-9 MPG seems low for what the OP described. What he didn't say (or perhaps I missed it) was whether he has an auto or 6-speed. If he has a 3.8L/auto, moving to 5.13s will improve his MPGs from what I've read.
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Old 02-26-2016, 02:22 PM   #12
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Sounds to me like you have a bad o2 sensor. My '11 2 door with 4" lift, 37s, 4.88s, and armor/tire carrier/etc averages around 12, calculated by hand. I dont split between city/hwy. Best Ive got was 14 and worst was 10. Normally is in the 12s though.
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Old 02-27-2016, 09:52 AM   #13
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My '08 JKUR with a RK 3.5" & 315/70/17 is getting around 13-14 mpg and it is my DD on highway & city roads.
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Old 02-27-2016, 11:09 AM   #14
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I mean, I guess I could see you getting a little better, like by a few miles per gallon, but I wouldn't think much. You are running large tires and very wide ones at that. My 285 Ko2s I know are pretty heavy, but with that size plus whatever wheels you are running you got at least 100lbs each at every corner. That is a considerable amount of weight to be pushing around, especially with the underpowered 3.8L engine. Couple that with off road tread design, high lift etc etc...you aren't looking at all gas mileage killers. Your engine is working extremely hard to spin those heavy wheels and tires, more power would help. If you are running a heavy foot, easing up can net you 2--3mpg more.
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Old 02-27-2016, 12:40 PM
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I'd be happy getting a few more MPG, really, that'd net be almost 40% increase over what I'm currently getting. That's a big increase. Going from 28-30mpg on a smaller car is a small gain, going from 8-9 to 11 or 12 is something I could live with. Even my modded Vette C5 gets better mileage around town! So does my '03 Denali XL, but not by much. The AWB, bus-like stature and several tons of weight will do that. Ouch.

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