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Old March 4th, 2006, 07:24 PM   #1
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Default 00 xj MPG?

ok so i slap on some new to me bfg m/t's last weekend, nothing big there just 235's also changed out the orginal plugs as the jeep has 110k on her now... also put in a K&N air filter at this time. went to the local autozone to have the check engine light checked and found out it was a for the fuel cap.. not being on tight enough but it is and the seal is good.. but if i leave it idle for more then 5-10 minutes you can smell fuel a little bit... always been that way since I got it 1 1/2 years ago. anyways since this my last two tanks I have only been getting about 230-240ish miles to the tank normally i get around 280-300. so why would new plugs and new air filter get me worse milage the tires are the same size just m/t instead of the street tires it had. I am wondering if I gapped them wrong as the old plugs when takin out were very worn out platinums with a huge gap.. I put in some champion plugs non plat.. would that really make that big a difference? thanks
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Old March 4th, 2006, 07:31 PM   #2
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M/T will drop your gas milage, not sure why but they do
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Old March 4th, 2006, 07:41 PM   #3
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M/T's are a heavier tire. That makes a big difference. I had 31" MTR's and when I put 31" TSL's on my Jeep my gas mileage dropped a lot. I can tell when I am driving and accelerating that the weight of the tire makes a big difference.
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Mine is just sloppy
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Old March 4th, 2006, 09:15 PM   #4
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Tire tread has a HUGE effect upon gas mileage. When i worked at Belle we had coustomers come back several times complaining about how their new tires get worse gas mileage. That is why there is a "street tire and Off road tires"
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Old March 4th, 2006, 09:21 PM   #5
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Not the tires..

Unless you drove 2 tankfuls since last weekend when you installed the tires..


Mileage on my XJ has dropped with the colder weather we've had in the last month or so, by 30-40 miles per tank.

The copper plugs shouldn't have hurt you, IMO, they are the best out there for most engines.

You might want to play with the gap size.

Don't still have the old plugs, do you? Could always swap them and check..
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Old March 4th, 2006, 09:45 PM   #6
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You would be amaized it does have an effect on the tread pattern of a tire... For instance if you run a TSL Super Swamper Bogger on the road compared to a Kelly Charger the Kelly tire would have a better rate of gas mileage. For road use the quieter the tire then better the mileage. That was a good moto we used at Belle Tire. It can also be several other things as noted above. It is a Jeep they do weird things, thats what makes em so darn fun.

Good Luck!
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Old March 4th, 2006, 11:34 PM   #7
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Yeah, I get worse mileage because of the cold weather as well, but the tires are the biggest killer.
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Old March 5th, 2006, 02:14 AM   #8
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any mud tire gets worse milage because of the resistance that it has compared to an all season tires, most all seasons have an almost continous tread, where if it was made of metal it would still be a perfect circle. a mud terrain or swamper is more or less a polygon, not a circle. 'large voids" are essentially flat spots on the tread, causes noise and friction.... look at a bike tire... a great knobby dirt tire vibrates on the cement.... a nice slick racing tire is super smooth and fast... Its the tires

also the cold weather my xj has been eating gas too
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Old March 5th, 2006, 03:41 AM   #9
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does anyone realize gas quality standard were lowered recently , this has been a few months but when this haoppened i saw my mpg's drop so I staarted using a higher octane and mpg's went back to normal but it could just be my junk try a tank of 92-93 octane and check the milage if it ts closer to normal see what the milage to cost ratio is on reg and prem if it is more cost effective use teh premimum
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Actually, the new rule of thumb is you should make your tires bulge slightly more then skooterbuilt when he's wearing his tight ladies pants
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Old March 5th, 2006, 08:42 AM   #10
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On my 00XJ the cold is what kills my gas mileage the worst. In summer I can get close to 280 miles on a tank in the winter i can get 230 to 240 miles at the most. This is with the same setup year around.
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Old March 5th, 2006, 09:13 AM   #11
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thanks for the help, was thinkin it might be the tires as well, just didnt think it would knock off 30-40 miles per tank... the weather out here in MD hasnt been to bad been in the 40's except for the storm we got few weeks back. I ran her damn near empty last night put in 17.5 gallons of super 93 in... we will see if it helps.. I am gonna guess no cause the higher the octane the colder the burn of the fuel is my understanding. Thena again I have a road trip back to MI this coming weekend might be a good chance for me to check it out there on the highway stretching her legs over the hills of PA...
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Old March 5th, 2006, 01:39 PM   #12
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Quote:
Originally Posted by R2JinaTJ
does anyone realize gas quality standard were lowered recently , this has been a few months but when this haoppened i saw my mpg's drop so I staarted using a higher octane and mpg's went back to normal but it could just be my junk try a tank of 92-93 octane and check the milage if it ts closer to normal see what the milage to cost ratio is on reg and prem if it is more cost effective use teh premimum
Hmm, I am gonna have to try that too.
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Old March 5th, 2006, 02:02 PM   #13
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My XJ's mileage is about 250 for a tankful, I am running BFG AT's.

I think my mileage has dropped significantly since the cold weather came, but like you said its running really rich too. Ironically I just put the same plugs in mine too.

I may replace the coolant temp sensor and or O2 sensor just to see if it makes a difference.

anyone know how much they run ?
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Old March 5th, 2006, 05:41 PM   #14
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There is a high pass and a low pass o2, the high passs i payed $75 i think. My jeep gets 11-16mpg, but i also have 4:88 gears 35"s and about 9" of lift so i think that is great for me lol about 230-240 a tank
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Old March 5th, 2006, 06:10 PM   #15
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Not really the colder burn with higher octane, it has to do with the rate of travel of the flame front.

Winter gas, though, is different than summer gas.

One of the main characteristics of motor fuel is the RVP (reid vapor pressure) value.

In summer, fuels with a lower RVP of about 9 are used. Having a lower vapor pressure means that at a given temperature, it will vaporize less. Since you have warm temps in the summer, you want a lower vapor pressure/volatility for the fuel, otherwise there would be a higher risk of vapor lock.

In the winter, its the opposite. RVPs as high as 15 are seen. If you used a low vapor pressure fuel in the winter, it would condense on the cold intake manifold runners and not atomize nicely.

In general, it seems that winter knocks about 2 mpg off most of us for city driving. Thats usually 30-40 miles on an XJ tank.

Highway wise, well, I managed 19.5 mpg on my last trip to west branch, in february..
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Old March 10th, 2006, 09:38 AM   #16
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trensetter, just thought i would let you know right about where i am at milage wise...

as of late (last four or five tanks) i have been getting right at 12.7-13.1(best as well as latest fill up) mpg in my everyday driving, city driving, which includes excessive use of the go pedal when the fire tones go off..so i aint doing to bad considering the 5 inches of lift, roof rack, 31's and 3:55's...and all kinds of wind sails on the roof...

hth

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