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regearing & MPG

L

lonecountryboy

NAXJA Forum User
Location
Oregon
Just put on my 31's and was wondering if regearing to 410's would up my gas mileage? Finally ran a couple of tanks through the new motor and checked the mileage on the last tank and it was only 14.5mpg on stock 355's. Was hoping for a little more than that.
 
I had a huge drop in mpg also. it's a combination of the added height of the jeep, tire height, and tire weight.
don't forget about the impact of the unsprung weight on the power of the engine, it's a big difference.
 
i regeared my 89 5sp w/31" tires and stock gearing(3.07) to 4.10 to gain back the power lost by changing the final output ratio of the drivetrain. i just happend to gain back about the 10% i lost in gas mileage. if its for gas mileage, would the cost of regearing make of for the lost mileage? if so, do you plan on staying with 31" tires and a new engine needs more miles to break in to get a true indication of what the mileage will be.
 
I regeared the speedo and it is only slightly off I think it is off by 2-3% and Yes I was going to stay with the 31's. The O2 sensor is brand new with the rebuild I am just wondering if it got fouled up with anything I guess I will check it and see if it is working properly. I am satisified with the power and everything but I do know that in order for it to be optimal it should be regeared to 4:10's I just didn't think the gas milage would drop that much.
 
why regear? pull 4.10 axles out of a 4cyl XJ. It'll cost less than (just the parts for) regearing, and is worth it in terms of mileage. If you choose to actually regear axles I would make sure you have a rear end capable of running larger tires and go to 4.56 to make it worth the money.
 
Did all 4cyl xj's have 4:10's? Didn't they have vacuum assist for the 4WD engage? Are the rear leaf springs the same width? Just wondering what kind of a pita it would be to do that.
 
The automatic 4 cyl's in the early years had 4.56, but most 4 cyl's have 4.10's yes.
 
Look for any year 2.5 / AX5 combo. They should have 4.10s. I found a set out of a '93 for my brother (D30 / 8.25 27-spline).

and x2 to what Cal said. I found two sets of those in the JY (D30/D35 combos). 2.5 / AW4.
 
Just cut and pasted this from my speedo gear install page to help others thinking about it. http://www.go.jeep-xj.info/HowtoSpeedoGears.htm
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​
 
Good info Gojeep.

More complete than I ever could have been.

For the record, if anyone cares, I run 4.88 with 35's. When I DD'd the jeep, I would get 18-19 mpg that way. Now that I only drive it a couple times a month, I'm on the throttle a lot harder and get about 16.

When i had 4.56 and 32's, I would routinely get 20 mpg.
 
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