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Old 07-24-2013, 02:02 AM   #1
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What to do, What to do?

1998, 4.0L, 130K, 2" OME, 245/75/17 BFG

The boss (wife) told me the other day she didn't seem to be getting as many miles to a tank as normal. Plan on changing the O2 Sensors (NTK), and Sparkplugs (AP985).

Anything else I should look into doing? All fluids are changed regularly.

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Old 07-24-2013, 03:56 AM   #2
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LT245/75R17 = 32" tire diameter. Is this tire size recent, and have you done anything to change the axle gearing, and is the gearing 3.07's or 3.73's? Lastly did you swap the speedometer gear when you fitted 32" tires?

Basicly we need to determine if the mileage is accurate. Stock Wrangler tires are delivered in several sizes, the smallest being 205/75R15, a 27" diameter tire. Your 32" tires are 5" larger and the speedometer error is as much as 18.5% unless the speedometer gear was changed for the new tire size.

Said another way, when you fitted 32" tires, you effectively geared your Jeep UP by as much as 18.5%, which slowed it down. The natural result of this is that the driver puts the pedal down more, and mileage plummets. But you can buy a lot of gas for the $1200 or so that re-gearing for larger tires will cost.

But with this information, at least you will know why your mileage went down.

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Old 07-24-2013, 08:27 AM   #3
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Not a recent tire change (Came stock with 30/9.5R15) . Correct 32 teeth speedo gear installed.
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Old 07-24-2013, 08:48 AM   #4
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Quote:
Originally Posted by BNotrab View Post
1998, 4.0L, 130K, 2" OME, 245/75/17 BFG

The boss (wife) told me the other day she didn't seem to be getting as many miles to a tank as normal. Plan on changing the O2 Sensors (NTK), and Sparkplugs (AP985).

Anything else I should look into doing? All fluids are changed regularly.
You should caculate the current MPG.

Here is how I did mine:

First thing to do is use a GPS and determine if your speedometer is accurate and if not the percentage it is off.

Now, fill the gas tank, record the odometer reading (or reset the trip odometer to Zero)

Drive the jeep until it is low, approx 1/8 tank.

Fill the tank and record the exact number of gallons it took to fill it.

Record the odometer reading and subtract the beginning mileage from it (or record the number of miles on the trip odometer)

Divide the number of miles driven by the number of gallons it took to fill it back up. DO NOT omit the tenths if there were any.

Now do this a number of times and average it out.

If your speedometer is off multiply the number of miles driven by the percentage it is off and add it to the number of miles driven before dividing by the number of gallons.
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Both as a commuter vehicle in NW, NJ
April 2012.downsized and now a 99er.
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Old 07-24-2013, 03:41 PM   #5
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Whatever, I see nothing to disagree about in the message above, but if you are today experiencing less mileage than you were before, with the same driver, the same tires, and in the same terrain, then I am ready to accept your claim (as long as the same technique is used to calculate mileage as before).

The basics would be:

1) Check for PCM trouble codes (usually but not always accompanied by a "Check Engine Light").

2) Check engine compression and if uneven, do a cylinder leakdown test to see whether you need an engine rebuild or just a valve job. (If at all unsure, pay a mechanic do do these things.)

3) If compression is good and there is no more than 10% variation from one cylinder to the next, then replace spark plugs, distributor cap and rotor, the PCV sponge in the air cleaner, and the ignition wires.

4) Still no fix? Suspect a plugged catalytic converter(s).
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Old 07-24-2013, 04:00 PM   #6
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I guess I am an anal old man, i have recorded every gallon of gas I purchased and the mileage for my wranglers since the first day I owned them. My YJ was used but my TJ was brand new. Since I commuted 50 miles each way every day MPG was important. When I was working, laid off last year, I was able to get a consistent 18-19 MPG by driving like the old man I am. Now that I do nothing but local driving I still get 17-18 driving that way.
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Retired my 94 YJ 4cyl 5spd @ 200K
Now in a 04 TJ 4cyl 5spd CV @120K+
Both as a commuter vehicle in NW, NJ
April 2012.downsized and now a 99er.
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Old 07-26-2013, 01:06 PM   #7
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With Jeeps, much of the "poor gas mileage" has always been associated with larger than stock tires. These effectively gear up the powertrain, resulting in sluggish acceleration. Then the driver puts his foot down harder to compensate, and mileage declines drasticly.

Still, you can buy a lot of gas for the $1200 or so it costs to re-gear for tire size.

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