Yet another Gearing Question

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12-02-2011, 08:28 PM

jsowards

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Yet another Gearing Question

I have done extensive searching and cannnot find a straight answer. I have a 98 XJ (4.5" Lift) with 3.55 Gears and 33x12.50 tires. My gas mileage sucks and its super sluggish especially on the highway. I have heard mixed things about gearing but am thinking 4.56 gears. I am looking to restore my MPG's and power. The AW4 trans seems to be searching for gears and downshifting often. What gears will improve my gas mileage? I can't take using a quarter tank of gas going to school and back 3 days a week.

Copied this from my searching according to this 4.56 would put me in the maximum efficiency and MPG range for the 4.0 motor however don't know how true this is:
Gearing for best MPG

Most people think that less revs will equal better MPG. That is not always the case as less revs can also mean more strain on the engine which causes more fuel use. According to a engine design text‚ maximum fuel economy occurs with open throttle and at a piston speed of 1200 – 1500 ft/min. So if you know the stroke of your engine‚ you can calculate the rpm for best fuel economy. Usually comes out somewhere between 2000 – 3000 rpm. Dino provided me with the following numbers. The 4.0L has a stroke of 3.411" and a 4.5-4.8L stroker has a stroke of 3.895" so the numbers would be as follows: 1200-1500ft/min piston speed = 2110-2640rpm on a 4.0, 1848-2310rpm on a stroker. So use the table above to pick the right gears for your tyre size. Above gives the average cruising speed of 65 mph or 105 kph. The more weight you carry the more revs you need to stop the extra fuel use through engine strain. The extra revs is also better for the auto and even engine temperature with less strain and the water pump running faster to circulate the coolant.
You can also use the gear chart to work out what axle ratio is needed to return you to stock gearing by finding your stock gear tooth number under the new tyre size. Below is also a guide to what revs you will have with different combinations. This is about what you will have in overdrive or forth in a auto, ( 0.75 ). Pre '91 will be slightly slower at 0.70, Peugeot ( BA-5/10 ) had a 0.72 and faster for a AX-15 manual at 0.79 or a AX-5 with 0.85.

RPM's at 65 MPH/105 KPH w/ 3.55:1 axle gears running 28's: 2,076 RPM
RPM's at 65 MPH/105 KPH w/ 3.55:1 axle gears running 30's: 1,938 RPM
RPM's at 65 MPH/105 KPH w/ 3.55:1 axle gears running 31's: 1,876 RPM
RPM's at 65 MPH/105 KPH w/ 3.55:1 axle gears running 32's: 1,817 RPM
RPM's at 65 MPH/105 KPH w/ 3.55:1 axle gears running 33's: 1,762 RPM
RPM's at 65 MPH/105 KPH w/ 3.55:1 axle gears running 35's: 1,661 RPM

RPM's at 65 MPH/105 KPH w/ 4.10:1 axle gears running 28's: 2,398 RPM
RPM's at 65 MPH/105 KPH w/ 4.10:1 axle gears running 30's: 2,238 RPM
RPM's at 65 MPH/105 KPH w/ 4.10:1 axle gears running 31's: 2,166 RPM
RPM's at 65 MPH/105 KPH w/ 4.10:1 axle gears running 32's: 2,099 RPM
RPM's at 65 MPH/105 KPH w/ 4.10:1 axle gears running 33's: 2,035 RPM
RPM's at 65 MPH/105 KPH w/ 4.10:1 axle gears running 35's: 1,919 RPM

RPM's at 65 MPH/105 KPH w/ 4.56:1 axle gears running 28's: 2,668 RPM
RPM's at 65 MPH/105 KPH w/ 4.56:1 axle gears running 30's: 2,490 RPM
RPM's at 65 MPH/105 KPH w/ 4.56:1 axle gears running 31's: 2,409 RPM
RPM's at 65 MPH/105 KPH w/ 4.56:1 axle gears running 32's: 2,334 RPM
RPM's at 65 MPH/105 KPH w/ 4.56:1 axle gears running 33's: 2,263 RPM
RPM's at 65 MPH/105 KPH w/ 4.56:1 axle gears running 35's: 2,134 RPM

RPM's at 65 MPH/105 KPH w/ 4.88:1 axle gears running 28's: 2,855 RPM
RPM's at 65 MPH/105 KPH w/ 4.88:1 axle gears running 30's: 2,664 RPM
RPM's at 65 MPH/105 KPH w/ 4.88:1 axle gears running 31's: 2,579 RPM
RPM's at 65 MPH/105 KPH w/ 4.88:1 axle gears running 32's: 2,498 RPM
RPM's at 65 MPH/105 KPH w/ 4.88:1 axle gears running 33's: 2,422 RPM
RPM's at 65 MPH/105 KPH w/ 4.88:1 axle gears running 35's: 2,284 RPM

Last edited by jsowards; 12-02-2011 at 08:38 PM.

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12-02-2011, 08:34 PM

jsowards

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I have heard mixed reviews some report 11mpg with 4.56 and 33's and others say 18+ (stock numbers).

Last edited by jsowards; 12-02-2011 at 08:43 PM.

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12-02-2011, 09:21 PM

xjmarc

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4.56 will take you back close to stock. Ran them with 33s, 35s and now with 38s but they are a bit lacking with the bigger tires but they were in the axles when I got them.

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12-02-2011, 09:37 PM

Gorillaxj

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I loved 4.56's with 33's. I like 4.56's with 35's. I would go 4.88's if you plan to go 35's.

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12-02-2011, 10:23 PM

jsowards

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How was your MPG with the combo?

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12-02-2011, 10:25 PM

Gorillaxj

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about 16 MPG with the 33's, and about 14 MPG with the 35's. with a corrected speedometer.

Last edited by Gorillaxj; 12-02-2011 at 10:28 PM.

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12-02-2011, 10:26 PM

Nickell586

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Baseline4x4.com has a gear/tire chart that will tell you. I'd send you the link but keeps sending me to the mobile site since I'm on my phone.

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12-03-2011, 07:52 AM

jsowards

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Thats a few MPG better than what I'm getting now and having the restored power would be nice. Highway driving with 3.55 and 33's is terrible I have zero passing power and feel like I could grow a beard faster than I could get to 70.

Last edited by jsowards; 12-03-2011 at 07:54 AM.

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12-03-2011, 10:13 PM

sgtskid

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Quote:

Originally Posted by Gorillaxj

about 16 MPG with the 33's, and about 14 MPG with the 35's. with a corrected speedometer.

X2 I have 4.56's with 33's and I get about 14-16 mpg. Gas mileage will still suffer...just because the thing still weighs as much as a

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12-03-2011, 10:20 PM

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a-gage

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with 33s I would go with 4.56 bit you will never get stock fuel mileage with a lift and 33s due to the extra rotating mass of the tires and the aerodynamic forces that come with the lift re gearing will not only help you mileage but will save your trans and engine because they wont be working nearly as hard

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12-03-2011, 10:26 PM

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Gorillaxj

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Quote:

Originally Posted by sgtskid

X2 I have 4.56's with 33's and I get about 14-16 mpg. Gas mileage will still suffer...just because the thing still weighs as much as a

yeah and i didn't always get 16mpg, sometimes I got 14mpg with 33's. just like how i sometimes get 12mpg with 35's and 4.56's... lame! lol

From what i've found its a lot more sensitive to having it loaded with things in the back more then it was when I had 31's and stock gears. also retrained my self to be easy on the throttle getting on the free way. burn a lot of fuel getting up to speed.

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12-05-2011, 02:44 PM

#12

jsowards

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Quote:

Originally Posted by Gorillaxj

Just like anything its going to be subjective to your driving habits. It is just difficult to not mash the skinny pedal when you are fending for your life merging into highway traffic. The 4.56 gears would restore some power which I'm sure would help raise my mpg seeing as I would no longer need to hit the gas quite so hard. I am pretty conservative when I drive the XJ. If I want to go fast I drive my other car and wind the Roots SC up