Worse MPG after re-gear. Any soloutions?

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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.
 
Assuming no other concurrent changes (you mentioned same tires) and that your calculations are correct with the new gears and speedo calibration, I wouldn’t expect a change that big. I went from 3.08 to 4.11 on 33s with a 4.0/AX-15 combo and dropped slightly (0.5 to 1 mpg) even though drivability improved greatly.

What specific differentials did/do you have before and after?

I think drivetrain parasitic loss is a more significant factor in TJs than some other vehicles. On a dyno, my TJ with the stock 4.0 put 137 hp to the wheels vs factory spec of 181 at the crank. That‘s 24% loss is vs 15-18% I’ve seen in other manual trans vehicles. With regard to differentials, I saw an unexpected 1-1.5 mpg increase when I went from an Eaton trutrac to an e-locker. I suppose all those helical gears drag more than an open (unlocked) diff. Also a 44 is physically bigger than a 35 and that may contribute to parasitic loss. Still, 25% ?
 
Assuming no other concurrent changes (you mentioned same tires) and that your calculations are correct with the new gears and speedo calibration, I wouldn’t expect a change that big. I went from 3.08 to 4.11 on 33s with a 4.0/AX-15 combo and dropped slightly (0.5 to 1 mpg) even though drivability improved greatly.

What specific differentials did/do you have before and after?

I think drivetrain parasitic loss is a more significant factor in TJs than some other vehicles. On a dyno, my TJ with the stock 4.0 put 137 hp to the wheels vs factory spec of 181 at the crank. That‘s 24% loss is vs 15-18% I’ve seen in other manual trans vehicles. With regard to differentials, I saw an unexpected 1-1.5 mpg increase when I went from an Eaton trutrac to an e-locker. I suppose all those helical gears drag more than an open (unlocked) diff. Also a 44 is physically bigger than a 35 and that may contribute to parasitic loss. Still, 25% ?

Everything was stock before. Dana 30/35. Now I have a dana 30 high pinion and a dana 44 in the rear. I went with Yukon gears, per the shop. It was a master kit, so most everything was replaced. Then Eaton E-lockers. It drives MUCH better now, but that is a steep loss of MPG. I also see multiple respected people in these forums saying they got better MPG after getting the engine in the right RPM band. I'm fine with it just staying the same even, lol.
 
Too many factors at play to make a determination.
Speedo calibration on before vs now.
New gears and bearings may be tight.
New brakes might be dragging. New alignment might be different.
Fuel quality changed, it does where I live at different times of the year.
Outside temperature is different.
Tire pressure might be different.
Or just loading the engine differently in overdrive.
 
Can’t believe everything you read.
I’ve never seen a taller gear provide better gas mileage in any vehicle, and I’ve been driving for almost 60 years.
Doesn’t relate.

I have regeared probably 200 vehicles or more when I was a tech. I have never seen more than a couple percent change from a regear. Most of the time the difference is within the range of error from tank to tank.
 
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I have regeared probably 200 vehicles or more when I was a tech. I have never seen more than a couple percent change from a regear. Most of the time the difference is within the range of error from tank to tank.
I can 100% verify I saw an unexpected increase from 11-somthing mpg, which never reached even 12 mpg, to over 14 mpg now since regearing from 4.88 to 5.38. My 4.0 with its 42RLE automatic was lugging on the highway with 35's with the 4.88 gearing which is why it got the terrible mpg. Regearing to 5.38 brought the rpms up enough to stop the engine from lugging so it operated more efficiently. No bullshit, it was 11 mpg before my regear and now a consistent >14 mpg since the regear. And yes I know precisely how to accurately compute my mpg.
 
As much as I'd like to bump to 4.56, I have a long commute & pretty happy with the 3.73 mileage. 17.6 mpg this last tank. 31s also.
Never regear to improve the mpg, you'd never recoup the costs of regearing with the better mpg. But if it's better performance from running a big tire then it's absolutely a good idea and from some combinations the mpg just might go up a tad too like it did with mine. :)
 
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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.

I wouldn't expect mileage to change much on either side with an axle re-gear. Only improvement to expect is in the driveability.

When was the last time you checked your O2 sensors?
 
I can 100% verify I saw an unexpected increase from 11-somthing mpg, which never reached even 12 mpg, to over 14 mpg now since regearing from 4.88 to 5.38. My 4.0 with its 42RLE automatic was lugging on the highway with 35's with the 4.88 gearing which is why it got the terrible mpg. Regearing to 5.38 brought the rpms up enough to stop the engine from lugging so it operated more efficiently. No bullshit, it was 11 mpg before my regear and now a consistent >14 mpg since the regear. And yes I know precisely how to accurately compute my mpg.

11 to 14 is only 3mpg. I can get that swing just changing driving style. I didn't say it was impossible to get a change, just not to expect that change to be massive.
 
Something is wrong somewhere. I was getting 16.5 MPG with 33's and 5:13 gears, and now same gears and 37's is still getting 14mpg. Granted I live in a relatively flat area but 12 MPG on 31's is ridiculous. My Armada on 32's gets 14 MPG and it's a V8 full size 3 row SUV.

I agree with above, check and see if your brakes are dragging, or if you've got drive line bind somewhere.
 
I wouldn't expect mileage to change much on either side with an axle re-gear.

I believe the physics agrees with you. In the end it takes the same amount of work to move the Jeep regardless of your gearing, so it should only make a discernible difference if either before or after the regear had you cruising in a condition that's way outside of the engine's efficiency band...like trying to cruise at 1500rpm.

The 42RLE could be a slightly different beast since the gearing could influence how much time you spend in TCC lockup or hunting between 3rd and 4th, which is likely to be more impactful than the gearing change in itself.
 
fwiw, I get about 13 with 35s, 4.88 and an AX15. 12 seems pretty awful for 31s.

How many miles have you put on it since the regear? I have to wonder if the gears were set up way too tight, but if that's the case I would expect them to eat themselves and be noisy in short order. When my first set of gears ate themselves in 2000 miles, the guy that set the new ones swore I would feel a difference in acceleration because of how poorly the first set had been done. I did, but it was hard to say it was a better setup or the fact that I also went from 4.56 to 4.88.
 
The 42RLE could be a slightly different beast since the gearing could influence how much time you spend in TCC lockup or hunting between 3rd and 4th, which is likely to be more impactful than the gearing change in itself.
My 42RLE never "hunted" between 3rd and 4th, it was damned hard to even get it out of 4th (Overdrive) once it was in it. You had to stomp on the gas hard to get it out of 4th or turn the OD off.
 
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My 42RLE never "hunted" between 3rd and 4th, it was damned hard to even get it out of 4th (Overdrive) once it was in it. You had to stomp on the gas hard to get it out of 4th or turn the OD off.

Mine was the same way until I tuned it with HP. I used to push the OD switch to get it to kick down. After the tune it drives much better.