After some reading, I found that mpg's are low for having 3" of lift and 30's. Average is just above 14 mpg (13 in winter). Watching the overhead, mpgs drop to 5 or 6 off the line, and that's babying it. Is this typical or is this a sign of bad o2 sensor, map sensor, vacuum leak etc.? Its a 98 4.0 at 115k so I'm debating doing a round of sensor changes, but don't wanna miss anything that could help/hurt. Thanks.

 

http://www.primechoiceautoparts.com/...-Symptoms.aspx

It would be best to test the sensors first. The problem you are having is what sounds like an 02 sensor.

 

A 14-15 year old Jeep with 115k mi is going to have a lot of worn out everything. O2s generally only run you about 80-100k.

That said there are a number of other contributing factors. Lift, tire size, tire pressure, tire mass, other weight on the Jeep, gearing, transmission type, air filter, catalytic converter, MAP, CTS, TPS, and winter fuel blend come to mind.

As well your overhead console's calculations are going to be off as your speedometer is no longer accurate due to the tire size change.

 

Best way to know what your gas mileage is. Top off the tank WRITE DOWN THE MILEAGE drive for a day. Go back the same pump top it off. Write DOWN your mileage again. Subtract the MILEAGE divide the gallons you just put in. Example on 2-14-13 the mileage was 120000. And on 2-15-13 you fill up again and the mileage is 120200. And you pumped 10 gallons of gas. After doing the math you'll know that your mpg is 20.

 

Gota remember his odo is not accurate with the 30" tires.

 

I have 265/65/16s on mine and for every 100 miles it shows on the odometer, it is actually 108.

 

Would it be an upstream or down stream o2 sensor?

 

Both effect fuel mileage do both not just one.... They are probably original with that year and mileage... And NTK is the best replacement besides mopar you can get hope this helps

 

This ^^^

My old gal has 230k on her, but I am still able to maintain 15mpg around town with a 4.5" lift and 31" duratracs. I did this by replacing, plugs, wires, distrib cap, button, old injectors to Bosch 703's, fuel filter, fuel pressure regulator, catalytic converter, air filter, O2 sensor (my year only has one), and the old exhaust manifold (replaced it with the APN stainless steel). There is more I could do to get better mpg yet, but these are the things that are less costly to replace and get good results.

 

Yup... on a vehicle of our age that's basically what a tune up is lol

 

Bingo. What these guys said.

Unless you are tracking your mileage as described above, with a known accurate odometer, or with the correct mathematical corrections for your tire size, you have no idea what your gas mileage is.

If you are going by the "miles per tank" method like so many do, your "calculations" are just lying to you. They can be WAY off.


Miles from a known accurate (or corrected for tire size) odometer divided by gallons pumped is the ONLY way to know what you really have.

Anything else is going to mislead you.

As for replacing sensors, yeah, if you want to throw money away on guesses. Bad sensors usually have other symptoms. Do you have any of those? Have you run any tests?

Blind parts-swapping is an expensive way to maintain a car. You never really learn anything that way, either.

In a pinch, when you MUST get running again, throwing some parts at it might be what you have to do.

But otherwise, you are just paying a stupid tax.