Worse MPG after re-gear. Any soloutions?

Take a nice long drive at highway speeds at least a mile or two and then pull over somewhere safe and feel the face of all the rims where the lug nuts are. If any of them are warmer at the center than they are out by the tire then your brakes arent releasing all the way. Driveline bind will be more of a vibration that you feel get worse the faster you go.

I mean it vibrates a bit at 70, but It doesn't feel like anything crazy though. Like it vibrates more than the 2017 nissan sentra I used to drive, but I think that is to be expected.
 
What was your cruise rpm before vs now? Mine is a stock '05, 3.73 gear with 33's getting a high of 16.8mpg.

Unusual for wheels to turn that freely with disc's, but at any rate that indicates brake drag is not your problem.
 
Last edited:
Hope you are measuring miles driven on a gps and not your odometer. The speedometer must be corrected when you change gear ratios.
 
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.

Hi, just curious. You didn't mention speedometer calibration. Was this also done and checked with a GPS ?? Sorry if I missed that detail.
 
  • Like
Reactions: Lunch Patterson
Update: I jacked up the Jeep, and spun all the tires. The back tires spin easy and keep going like a merry go round after you stop. The front ones do not though. It is more work to get them spinning, and they almost stop right away. I don't think the brakes are dragging, but I guess they wouldn't make noise necessarily. I tried them both in 2h, and 4h. What are some things to look for up front that might be giving that extra resistance? I'll also try a long drive and feeling the brakes, and if they are warm/hot.

I mean it vibrates a bit at 70, but It doesn't feel like anything crazy though. Like it vibrates more than the 2017 nissan sentra I used to drive, but I think that is to be expected.

my 2004 on 37's will sail down the highway at 90 mph running so smooth I usually don't realize I'm going that fast until I look at the speedometer and slow down. Any vibration means something... it might not be the cause of your poor mileage, but vibrations are caused by something and can usually be cured if you decide you want to cure them. As far as mileage goes, when was the last time you gave your Jeep a tune up?
 
Hi, just curious. You didn't mention speedometer calibration. Was this also done and checked with a GPS ?? Sorry if I missed that detail.

Yeah, it was done both times with a gps/odbII reader that calculates. I also did it by hand and I'm not too far off it. The speedo is within 1 mph give or take.
 
my 2004 on 37's will sail down the highway at 90 mph running so smooth I usually don't realize I'm going that fast until I look at the speedometer and slow down. Any vibration means something... it might not be the cause of your poor mileage, but vibrations are caused by something and can usually be cured if you decide you want to cure them. As far as mileage goes, when was the last time you gave your Jeep a tune up?

At 100k when I got it. A bit over a year ago. The steering does feel a bit "loose", but I figured it was just how a solid axel feels. I need to find someone else's Jeep to drive, so I know what is bad, or normal. 37's with a stock engine going 90 with it being smooth sounds like either exaggeration, or you dropped an LS in there, lol. Even when this had 29" tires it had to struggle to go up a steep hill on the highway.
 
  • Like
Reactions: SPECWAR
Never. I figured they would throw a code. I guess they could just happen to go out exactly the same time I did an axel swap.

I recommend you check their condition. Mine needed replacing after 62k. I noticed a drop in mileage and the trims were not reading as they should be. I chose to replace all 4 and I could tell a very clear difference afer that.
 
I recommend you check their condition. Mine needed replacing after 62k. I noticed a drop in mileage and the trims were not reading as they should be. I chose to replace all 4 and I could tell a very clear difference afer that.
I might as well. I don't know that they have ever been replaced. I figured it was something with the axels, since it started when they got switched. What "trims" do you mean are not reading as they should?
 
What "trims" do you mean are not reading as they should?

fuel trims (the PCM adjusts the fueling based in part on feedback from the O2 sensors) - you can check/read them with an inexpensive OBDII dongle and a free app like Torque Lite
 
  • Like
Reactions: psrivats
If we simplify everything by taking all resistance variables away, lower ratio will not yield better efficiency, never did, never will. Faster engine spins, more fuel it burns , aka, more energy you consume to do same work = same work done with more energy wasted.

Increase rpm = burn more fuel
increase resistance = burn more fuel
At some point, due to change in some variable, there will be a crossing point and one of them will burn less than the other to produce the same result.

You cant compare and expect same result if you are not comparing apples to apples.
For someone else that crossing point was else where for a different reason.

example:
Jeep A had too much resistance due to larger tires, higher lift, and it driving 75mph. By increasing rpm, benefit overcome resistance, and burn less fuel.
Jeep B, same exact copy of Jeep A, but never went faster than 45 mph. By increasing rpm it started to burn more fuel, because at 45 mph there is only a fraction of resistance in comparison to 75 mph.
 
fuel trims (the PCM adjusts the fueling based in part on feedback from the O2 sensors) - you can check/read them with an inexpensive OBDII dongle and a free app like Torque Lite

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?
 
I also made the switch from 3.73 to 4.56, and also have 4.0, 42RLE 2004 sport, and also 31x10.5xR15 (Goodrich KO2).
I have to say that my MPG is worse now, but it is only because now I drive it faster, accelerate faster etc.
I get now 2500RPM at 70MPH, which high enough to provide proper power, and now I can drive 80MPH w/o a problem (highway has 75MPH limit, so everybody drives 80MPH).
4.56 with 31 does not feel anemic at all, it is really responsive, to the level that it can be.
I can try to measure my MPG, but I doubt that it is 12MPG if I drive it normally.
 
  • Like
Reactions: Lunch Patterson
I also made the switch from 3.73 to 4.56, and also have 4.0, 42RLE 2004 sport, and also 31x10.5xR15 (Goodrich KO2).
I have to say that my MPG is worse now, but it is only because now I drive it faster, accelerate faster etc.
I get now 2500RPM at 70MPH, which high enough to provide proper power, and now I can drive 80MPH w/o a problem (highway has 75MPH limit, so everybody drives 80MPH).
4.56 with 31 does not feel anemic at all, it is really responsive, to the level that it can be.
I can try to measure my MPG, but I doubt that it is 12MPG if I drive it normally.

It would be good to hear your mpgs! I seem to get 12 to 12.5 no matter how I drive it. Easy, hard, highway, city. It does definitely drive much better and doesn't bog anymore.
 
At 100k when I got it. A bit over a year ago. The steering does feel a bit "loose", but I figured it was just how a solid axel feels. I need to find someone else's Jeep to drive, so I know what is bad, or normal. 37's with a stock engine going 90 with it being smooth sounds like either exaggeration, or you dropped an LS in there, lol. Even when this had 29" tires it had to struggle to go up a steep hill on the highway.
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?