Average 8 mpg, need suggested fixes

[Brian83]

Got a Rubicon. I wonder if the Autel at work has this function hmmm

You need a SpeedoHealer to calibrate the signal coming from you VSS to account for the larger than stock tires.

 

[Rubicon John]

You need a SpeedoHealer to calibrate the signal coming from you VSS to account for the larger than stock tires.

In the how-to write up one option listed was for the dealership to reflash the ECU to program the different tire size. It seems with the correct software this would be possible. I can do it on the vehicles I work on, just not sure if the Autel has the capability.

 

[Thatguy]

When i bought my Tj last fall, it ran but it ran like crap. Sluggish and felt choked up. Jeep was used to running the city streets of Chicago burbs, probably never saw speeds higher than 35mph. I didnt run it long enough to get a mileage check, just automatically gave it a well deserved tune up. I also ran a few bottles of Seafoam thru the fuel system. I didnt see the results instantly, more gradual if anything. When i first bought it and drove it around the rural roads where i live, it seemed to struggle leaving from a stop, very labored. I went thru fuel rather quickly. Things didnt improve immediately after the tune up either. I noticed that the more i drove it at highway speeds, the better it started performing. Ive probably run 2 full bottles of Seafoam thru it since i got it. Not saying this is a miracle cure or anything, but with some decent fuel it seems to have made a big difference in how my Tj has been running. I am consistently getting 12-14mpg with the winter blend fuels we use in WI this time of year. Id say it will go up to 14-15 by summer

99 Sport 4.0l automatic w/stock gearing (assuming) and running 33s, low miles at 93K

For comparison, my 12 JKU w/35s gets 18mpg and w/265s gets 20mpg

I dont own Jeeps for their stellar fuel savings, i have a diesel truck for that

 

[Brian83]

In the how-to write up one option listed was for the dealership to reflash the ECU to program the different tire size. It seems with the correct software this would be possible. I can do it on the vehicles I work on, just not sure if the Autel has the capability.

Im 99.9% sure the dealers that we able to "re-calibrate" rubicon speedometers were using a converter box. There is no where in the PCM program to enter tire size.

 

[Rubicon John]

Im 99.9% sure the dealers that we able to "re-calibrate" rubicon speedometers were using a converter box. There is no where in the PCM program to enter tire size.

Gotcha. I will look into it, although admittedly I haven’t had to get into the computer functions in a TJ yet.

 

[Painfultruth]

Im 99.9% sure the dealers that we able to "re-calibrate" rubicon speedometers were using a converter box. There is no where in the PCM program to enter tire size.

For a Rubicon there are tuning parameters within the PCM specific to Speedo calibration. With the correct software no additional device is required to recalibrate.

Some tuning devices (Diablo etc.) on the market have that functionality included as well.

 

[Brian83]

For a Rubicon there are tuning parameters within the PCM specific to Speedo calibration. With the correct software no additional device is required to recalibrate.

Some tuning devices (Diablo etc.) on the market have that functionality included as well.

Not saying your wrong but why would that be a Rubicon specfic thing when if you do a Super Short Slip Yoke Eliminator and switch to a Hall Effect sensor just like the Rubicon and it's plug and play? If you can do it to a Rubicon you should be able to do it for any TJ. I know the option isn't there in HP Tuners for early TJs and I've never heard of anyone saying it's there on later TJs.

 

[Painfultruth]

Not saying your wrong but why would that be a Rubicon specfic thing when if you do a Super Short Slip Yoke Eliminator and switch to a Hall Effect sensor just like the Rubicon and it's plug and play? If you can do it to a Rubicon you should be able to do it for any TJ. I know the option isn't there in HP Tuners for early TJs and I've never heard of anyone saying it's there on later TJs.

I can’t speak firsthand about JTEC PCM’s, but the tuners for the NGC specifically state tire size calibration functionality in the product literature and my tuner has the ability to do it. So the ability does at least exist for newer TJ’s.

 

[Brian83]

I can’t speak firsthand about JTEC PCM’s, but the tuners for the NGC specifically state tire size calibration functionality in the product literature and my tuner has the ability to do it. So the ability does at least exist for newer TJ’s.

NGC is only the last two years of the TJs so that would be a limited number of vehicles compared to overall.

 

[Painfultruth]

What tuner do you have? I'm just curious.

Diablo I3

User Adjustable Parameters
Idle RPM
WOT Fuel
Rev Limit
Speed Limit
Idle MDS
Throttle Sensitivity
Tire Size
WOT Spark
Cooling Fan Temp

 

[Rubicon John]

Diablo I3

User Adjustable Parameters
Idle RPM
WOT Fuel
Rev Limit
Speed Limit
Idle MDS
Throttle Sensitivity
Tire Size
WOT Spark
Cooling Fan Temp

Interesting. I have a Diablo I3 also, I’d just have to buy the Jeep licensing for it.

 

[HardSell]

I currently get about 8-9 miles per gallon,

Inexcusable. Replaced my stolen cherished, somewhat built '87XJ: 410s; 4" lift; ARBs; 31KOs; Heavy bumper; Heavy trussed unibody stiffening; Dana 44 rear; 280,000 miles wheeled hard, untouched 4.0 5sp (Yeah, the PU tranny). Last mileage check before stolen was 15 in town, 21 at 70-80 on interstate with camping gear on top. There's a reason FCA returned to part time 4WD regarding the JLs, once again claiming 16-23mpg. Just replaced with an '05 Rubi 6sp with 33s and short arm. 15 in town, but 11-12 on interstate. I'm gonna remove front shaft, drive a hundred freeway miles just to see what happens.

 

[reddvltj]

Inexcusable. Replaced my stolen cherished, somewhat built '87XJ: 410s; 4" lift; ARBs; 31KOs; Heavy bumper; Heavy trussed unibody stiffening; Dana 44 rear; 280,000 miles wheeled hard, untouched 4.0 5sp (Yeah, the PU tranny). Last mileage check before stolen was 15 in town, 21 at 70-80 on interstate with camping gear on top. There's a reason FCA returned to part time 4WD regarding the JLs, once again claiming 16-23mpg. Just replaced with an '05 Rubi 6sp with 33s and short arm. 15 in town, but 11-12 on interstate. I'm gonna remove front shaft, drive a hundred freeway miles just to see what happens.

2010 Ram 3500 Diesel Crew Cab long box... My father and I replaced the unit bearings with manual hub conversion and it made about 1 to 1.5 MPG increase on the open high way. I doubt the Jeep is going to benefit that much because aerodynamics are what holds the jeep back on MPG

 

[CharlesHS]

Wondering if the OP has made any progress to figure out why his TJ has such low MPG....

 

[HardSell]

I doubt the Jeep is going to benefit that much because aerodynamics are what holds the jeep back on MPG

I understand your point, but I gotta see. The TJ's long hill climb persistence is impressive, much better than the old XJ which I often had to go down to 4th, but still netted 20+mpg sans wind. TJ runs about 400 more RPM at FW speed. My brother's '06 TJLR 6sp with 33s got 20 on highway. Financials forced him to sell. Now he has a built FJ Cruiser. Rode with him in San Juans. 20 MPG! I wanna figure this out. Would be satisfied with 18 at 65-70mph. I do love the short wheel base. The 4.0 is granite solid and doesn't leak....yet.

 

[pagrey]

I wanna figure this out. Would be satisfied with 18 at 65-70mph.

This is not going to happen. Even a totally stock TJ will struggle to get 18 consistently at these speeds. On 31s I average 16.5 and consider myself the upper end of what is possible. Sure, one trip I got some good numbers but average, no way.