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Please school me on gears

kdailey4315

kdailey4315

NAXJA Forum User
Location
Pacifica, CA
I've read through some of the gear threads but couldn't find my answer. Why does not regearing when you put a lift on and get bigger tires lower your gas milage? I understand the aerodynamics of lifting and more drag but why does regearing get you "back" some of the lost gas milage?
 
because you have to use more throttle to achieve the same amount of go, and more throttle = more gas in. people also don't usually consider the change in wheel diameter for distance on the odometer, and without considering that, it will undoubtedly lower your mileage.
 
Gas mileage isnt ALL about RPMS. It has to do with throttle position as well. When you push that excelerator pedal, it opens the butterfly in the throttle body, letting in more air. To keep a good mixture, there has to be more fuel added along with the air.
Now think about when you are climbing a hill in OD. You have to keep adding more and more skinny pedal until it down shifts out of OD then you can let off the gas alittle. In OD you were bogging the engine. You may have been turning low RPMs but you had the pedal to the metal to maintain speed. Lower gearing will allow you to climb that hill without bogging the engine as much
 
Ray H said:
Gas mileage isnt ALL about RPMS. It has to do with throttle position as well. When you push that excelerator pedal, it opens the butterfly in the throttle body, letting in more air. To keep a good mixture, there has to be more fuel added along with the air.
Now think about when you are climbing a hill in OD. You have to keep adding more and more skinny pedal until it down shifts out of OD then you can let off the gas alittle. In OD you were bogging the engine. You may have been turning low RPMs but you had the pedal to the metal to maintain speed. Lower gearing will allow you to climb that hill without bogging the engine as much
Click to expand...

i wish there was an i con for hitting the nail on the head cuz it would apply here.
 
MiNi Beast said:
Hence gears lower your rpm's which in turn gives you better gas mileage.
Click to expand...

After putting on bigger meats your RPMS will drop some. Compensting that by swapping out stock gears for lower gears will actually RAISE your rpms.
 
waxer said:
After putting on bigger meats your RPMS will drop some. Compensting that by swapping out stock gears for lower gears will actually RAISE your rpms.
Click to expand...

ok so when I put my bigger meats on and had to push the gas more to get up to speed my Rpms dropped, but when I put my 4.10 gears in my rpms raised?

I run at about 2000rpms right now with 33's and 4.10's going 60mph. with out the gears I was runing 2600-3000rpms at 60, so I'm I not understanding the idea here or what?
 
asp387 said:
i wish there was an i con for hitting the nail on the head cuz it would apply here.
Click to expand...

Thank you, I wasnt sure I was explaining very well.
Back to the subject.
The terrain you're driving on matters alot when talking about gearing. If you are a flat lander, you will probably see better MPG with higher gearing and vice versa if you live in the mountains. Thats why when as a nationwide group we get together and discuss fuel mileage, you will see all kinds of answers. Some will tell you to run higher gearing and some will tell you to run lower for better MPG. It depends alot on how and where you drive.
 
MiNi Beast said:
ok so when I put my bigger meats on and had to push the gas more to get up to speed my Rpms dropped, but when I put my 4.10 gears in my rpms raised?

I run at about 2000rpms right now with 33's and 4.10's going 60mph. with out the gears I was runing 2600-3000rpms at 60, so I'm I not understanding the idea here or what?
Click to expand...

Without gears you were probably not running in OD, if you have an auto.

Once you got gears it allowed the trans to shift into OD and drop your Rpms.

-Alex
 
:doh: Alright I got it now, read everything again, makes since now, little slow need to read things three times. But I got you now, I was thinking the same thing but guess I didn't say it that way, but that was why I posted that linked, seemed to explain it better. Drink another one huh?
 
kdailey4315 said:
I've read through some of the gear threads but couldn't find my answer. Why does not regearing when you put a lift on and get bigger tires lower your gas milage? I understand the aerodynamics of lifting and more drag but why does regearing get you "back" some of the lost gas milage?
Click to expand...

Hmmm... lots of interesting answers in this thread, here's mine.

Every engine has an rpm range in which the engine is running at peak efficiently, some time refered to as the powerband. If you throw a set of 35 inch tires on with stock 3.55:1 gears, you rpms at any speed drop down to the point where engine is not running in it's most efficient rpm range, fuel economy and performance suffer. Re-gearing to 4.56:1 gears will raise the rpms at any speed back to near stock. You will won't achieve the same gas milleage as stock because big tires will increase rolling resistance, and a lift will decrease aerodynamics, but you can get most of your lost mpg back. Lowering the gears to much will cause excessive rpms beyond what is required and you will have decreased milleage again.

Ideally at normal cruising speeds you will be operating you engine at its most efficient rpm, which is usually on the lower part of the powerband.
 
Last edited:
flexdout said:
I must have missed the memo stating the aerodynamic qualities of an XJ?
Click to expand...

A lift, in effect, makes your Jeep bigger. More frontal area, more wind turbulance.
 
MiNi Beast said:
ok so when I put my bigger meats on and had to push the gas more to get up to speed my Rpms dropped, but when I put my 4.10 gears in my rpms raised?

I run at about 2000rpms right now with 33's and 4.10's going 60mph. with out the gears I was runing 2600-3000rpms at 60, so I'm I not understanding the idea here or what?
Click to expand...


You were in third gear.
 
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