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Old 07-06-2009, 09:52 PM   #1
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i got a 98 4.0 sport soft top. everything stock but 31x10.5 BFG all terrains. and a borla exhaust. 2 years ago when it was stock everything i used to get 300 per fill up. now im barely reaching 200. this was happening before i changed my exhaust. i know new tires effect mpg but 100 miles? i changed the air filter and oil regularly. even redid my cap and rotor as well as new spark plugs and wires. any tips or anyone with the same prob? i took off my doors and top to take off some weight but still no results.. helppppp

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Old 07-06-2009, 09:55 PM   #2
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did you replace the cat or no? The cat could be bad. IF you did then something is not reading right. your tires will toss the mileage down a hair but not 100 miles unless you put in the new speedo gear.

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Old 07-06-2009, 09:55 PM   #3
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i may be wrong, but i think taking the doors off actually hurts mpg, not helps - I know it saves weight, but I think the additional air resistance would more than cancel out the gain.
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Old 07-06-2009, 09:58 PM   #4
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i didnt change the cat.. but my mpg was just as bad before i changed my ex.. i dont know what else to do..
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Old 07-06-2009, 11:18 PM   #5
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Uh, did the change happen when you changed the tires?

The speedo gear is about the size of a quarter. The tires an extra large pizza. The difference is huge.

Best way to tell is to GPS it. The difference (between the odometer and the GPS) will be telling. Also, don't judge gas mileage by the "I get xx miles per tank". Start off filled and do the math (gallons divided by mileage). Sounds goofy but TJ gas gauges aren't very accurate and get less and less accurate the older then get.

Check it before you start throwing hard earned money at a potentially non-existent problem.
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Old 07-07-2009, 06:34 PM   #6
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do i have to change out the speed gear? and if so how? but im pretty sure the change was before my new tires.. thanks for the reply!
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Old 07-07-2009, 06:40 PM   #7
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Bigger tires without also correcting the speedometer will give the illusion of worse mpg since the odometer reflects fewer miles than actually driven. Then again if it's that bad, which yours sounds worse than that, it could be a bad upstream 02 sensor. When it goes bad, and a bad 02 sensor does not always trip the Check Engine light or generate a code, it causes the fuel mixture to turn rich which will reduce your mpg substantially.

Consider the 02 sensors as items that occasionally need to be replaced, they always do eventually go bad.

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