Worse MPG after re-gear. Any soloutions?

Ah, got ya. I have an odbII reader and know where fuel trims are. I'll look up what they are supposed to be. Sounds like if they are off O2 might be a thing to look at. Would injectors also possibly be a thing?
The re-gear and MPG change might be a case of “true-true-unrelated”. Or the “post hoc propter hoc” fallacy.

I had a similar change in MPG and it was fixed by new O2 sensors and a new air cleaner filter.
 
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The re-gear and MPG change might be a case of “true-true-unrelated”. Or the “post hoc propter hoc” fallacy.

I had a similar change in MPG and it was fixed by new O2 sensors and a new air cleaner filter.

Fair enough! Just an odd coincidence. I'm looking into the O2s and have a clean air filter. Anything to look at if they need replaced, or just might as well do it. Previous owner has all the paper work and I don't see O2 change in there anywhere.
 
That doesnt sound right. I've heard people say their Jeep can barely do 60mph etc.. But even the two 4 cyl Jeeps I've owned never struggled that bad. Do you rev your engine past 4k RPMs? Or do you upshift at like 2500 normally?

I have a 42rle so auto. It would do 90 if I made it. It does 75 "comfortably", until a big hill. It runs around 2700 at that. It won't unlock the torque converter at those speeds it will only downshift to 3rd and that puts it around 4200. I normally give it the amount of gas that keeps it shifting at 2500 ish.
 
I have a 42rle so auto. It would do 90 if I made it. It does 75 "comfortably", until a big hill. It runs around 2700 at that. It won't unlock the torque converter at those speeds it will only downshift to 3rd and that puts it around 4200. I normally give it the amount of gas that keeps it shifting at 2500 ish.

Taking into account that we have the same setup (4.0, 42RLE, 4.56 and 31s), you should not expect it to go uphill at 75MPH uphill. In order to go uphill at 75, it needs to be at least at 3000RPM, then maybe it will have enough power. This would require something like 5.13 or 5.38 gears and it would burn gas like crazy.
The existing setup is a good compromise between gas consumption and power.
 
Fair enough! Just an odd coincidence. I'm looking into the O2s and have a clean air filter. Anything to look at if they need replaced, or just might as well do it. Previous owner has all the paper work and I don't see O2 change in there anywhere.

I changed mine at ~70k; I’m pretty sure they were original.

If you think there sensors have >50k miles on them, I’d just replace them.

Make sure you read the Official TJ O2 Sensor sticky if you haven’t already.
 
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Look at the curve. 2750 is pretty much the best spot to be at. It is where the Torque meets the HP. Increasing RPM beyond it, would reduce the available torque, while increasing the HP.

E3F7BFC9-89FD-483D-8CA8-3C9574F66856.png
 
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Hey all. I upgraded the Jeep and went from 30/35 axels to a 30hp/44. I installed lockers and got a re-gear from a reputable shop. Before the re-gear the engine lugged all the time. I basically just ran it as a 3 speed. I have the 42RLE. The Jeep runs much better in the rpm band, but I lost around 4 mpg. I saw all these threads about getting better MPG, not worse with a re-gear. I still have the same tires. Just 31x10.50x15. I went from 3.73 to 4.56 gears. I averaged around 16 before and now it is at 12. I gave it a few months to make sure. The speedo is correct. I also double checked with a gps/odbii app that doesn't use the speedo to make sure. Does anyone have any ideas on things to check, or something that might be wrong now, before I start throwing money at it? I've tried a few tanks with barely using the skinny pedal and it didn't change anything. I've shaken the wheels to check the bearings and turned the wheels by hand to make sure nothing is dragging. I also had the shop double check everything, and they said it is all good. Any ideas? I know Jeeps aren't MPG machines, but 12 mpg with only 31" tires doesn't quite seem right from the other info I've seen on here.

Driving less is the best way to save gas.

12mpg on 31's is about 1-2mpg less than what I got on 31's.
 
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Ah, got ya. I have an odbII reader and know where fuel trims are. I'll look up what they are supposed to be. Sounds like if they are off O2 might be a thing to look at. Would injectors also possibly be a thing?

Start with the O2 sensors first and then you can go deeper into the rabbit hole. Keep it simple and proceed checking things in a logical manner. If we talk hypotheticals then you can ask questions like "what about my cylinder walls? what about my compression? what about X? What about Y?" and so on and so forth.
 
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I assume both sensor 2's need to be giving me some information. There are the trim readings from the odb2. No idea what any of this means. I've got O2 sensors on the way to do in 2 days. I'll update then!

fueltrim.jpg
 
I have a 42rle so auto. It would do 90 if I made it. It does 75 "comfortably", until a big hill. It runs around 2700 at that. It won't unlock the torque converter at those speeds it will only downshift to 3rd and that puts it around 4200. I normally give it the amount of gas that keeps it shifting at 2500 ish.

I don't have any experience with the Auto TJ on the street. All of my TJ's except for 1 were stick shift. The 1 I had with an auto trans was on one ton axles and 40's and never saw any street use. I hope you get it sorted out though with gas prices being up like they are.
 
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Did you try to do some basic treatment of the engine with the BG EPR? It is a can that you pour into the engine oil then run 10 minutes at 1300RPM in park and then do oil change. I see that the engine feels much better with it. I did the EPR like 3 times in last 2 years and really improved the feeling.
You might also want to spray the sea foam into throttle for burning all the gunk.
Then you take BG 44K which is gasoline additive and pour it into tank. Add to full tank and run through it.. it cleans the injectors.
Also you might want to replace the plugs.
I did all the above on my TJ. Preventative healthcare kind of.
 
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I assume both sensor 2's need to be giving me some information. There are the trim readings from the odb2. No idea what any of this means. I've got O2 sensors on the way to do in 2 days. I'll update then!

View attachment 372167

Certainly no expert but the short term & long term trims for each bank should ideally add up to 0 so something doesn't look right. Had to chase down two bad injectors in the wife's MKZ so I learned a little bit about it. Did the 02 sensors also which was probably unnecessary but its an '08 with 129k miles.
 
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Certainly no expert but the short term & long term trims for each bank should ideally add up to 0 so something doesn't look right. Had to chase down two bad injectors in the wife's MKZ so I learned a little bit about it. Did the 02 sensors also which was probably unnecessary but its an '08 with 129k miles.

I have seen O2 sensors that were inaccurate from the factory(Ford). A very well known tuner I worked with suggests that they be changed as a regular maintenance item.
I don’t feel you were wrong to change yours at 129,000.
 
I have seen O2 sensors that were inaccurate from the factory(Ford). A very well known tuner I worked with suggests that they be changed as a regular maintenance item.
I don’t feel you were wrong to change yours at 129,000.

Yea she was getting a generic bank 2 lean code. Ended up being two bad fuel injectors but I'm glad I changed the O2 sensors while I was at it.
 
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The short and long term trims can be off a bit. It's more concerning if they are off by more than 10%. Engine wear, fuel quality, altitude differences, fuel pressure among other factors are what the trims are adjusting for. Getting into the 20s and 30% range will result in a fuel system rich or lean code.
 
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Did you try to do some basic treatment of the engine with the BG EPR? It is a can that you pour into the engine oil then run 10 minutes at 1300RPM in park and then do oil change. I see that the engine feels much better with it. I did the EPR like 3 times in last 2 years and really improved the feeling.
You might also want to spray the sea foam into throttle for burning all the gunk.
Then you take BG 44K which is gasoline additive and pour it into tank. Add to full tank and run through it.. it cleans the injectors.
Also you might want to replace the plugs.
I did all the above on my TJ. Preventative healthcare kind of.

I put in Lucas fuel treatment pretty regularly. I pulled apart the IAC and throttle body and cleaned it when I got the Jeep. I did plugs about 16,000 miles ago.
 
I assume both sensor 2's need to be giving me some information. There are the trim readings from the odb2. No idea what any of this means. I've got O2 sensors on the way to do in 2 days. I'll update then!

View attachment 372167

I don't know what obdii scanner you are using, but here is some information for you - read the short section about O2 sensors. I'm no expert, but when I had the issue on the past, I requested my mechanic to explain it to me and he showed me the live data from his scanner, both with old and new sensors. It was great to see and get an idea.

https://obdsoftware.force.com/s/article/How-to-read-OBDII-live-data-A-mechanic-guide