Gas Mileage Tanks

Hi, I am looking to improve my gas mileage. I recently switched to the TrXus STS radials from InterCo, and I realize this has affected my mileage, but it really feels like I have a pinhole in my fuel tank.

Any suggestions for improving mileage? I plan on changing/upgrading the air filter and I have heard that a new fuel filter can help tremendously, but I want to be sure I know all my options.

So, any suggestions are welcome. Filter suggestions, other ways to improve...etc etc....

If you changed to a bigger tire than you had before without changing gears then the engine has to work harder to move the wheels. That could be your problem.

A hi-flow intake, cat-back exhaust, new cat, new headers and downpipe, or maybe an adjustable MAP sensor could help if you did change gears, or simply replaced tires with the same size.

keep in mind that if you increased the size of your tires and didnt re-gear accordingly, your speedometer/odometer is off now... not that they are accurate in the beginnig.. in all honesty the best way to increase the fuel efficiency of a properly tuned 4.0L is to use a lighter throttle foot.... i consitantly get 16-17 mpg in my 99.... i have a heavy foot and run the highways around 75 mph..... if i let my friend borrow it for a road trip he gets 21 mpg!!!!! his grandmotherly driving style pays off and i have tried hard to keep the foot out of it but i cant... sounds a lil old-fashioned but "slow down"..

These things drink gas and always will. You could spend $1000 in modifications to improve miledge and you might get 1 more mile per gallon, if not worse fuel economy. They are tuned from the factory for maximum miledge, so I would keep the drivetrain as bone stock as possible. Don't pay attention to that nonsense that a flowmaster or K&N will improve miledge. It won't and it may hurt it. My girlfriends stock XJ gets 2 or 3 mpg better than mine and I have all that garbage on mine. Your miledge drops drasitcally with every mph over 65, so try to cruise at that speed on the highway. If you are blowing down the road at 85, you are always going to be draining that tank faster.

HighOctane --

Let's start by asking what your mileage is now ... and if that's accurately calculated including any correction for non'standard size tires, or if it's just a guess because you feel like you're using a lot of gas.

A stock XJ should get somewhere between 16 and 19 MPG in around town driving, and 19 to 22 MPG highway (less if you cruise above 70 MPH). Unless your mileage is significantly worse than those figures, any attempt to improve it with toys is a waste of your time and money. As already commented, you could spend hundreds of dollars to improve 1/2 MPG, and your payback period will be measured in decades.

If you went with bigger tires, say 235's or 30's or bigger and did not regear keep it out of 5th if you have a 5 speed, don't know about automatics. I keep my 98 in fourth till I get above 65 and then go into 5th, 5th never gets used till I am on an interstate. Also how many miles on it ? It seems one of the biggest contributors to bad milage on a vehicle in good shape, recent tune up, etc is from a borderline O2 sensor. They have a big impact on milage. I'm one to talk, I just filled out my registration renewal and added up my new odometer milage and my old odometer milage difference, I now have a tad over 234,000 mi on mine and am still running the original 02's...

my 89 limited just got 21.5 mpg towing a trailer thought the canyon and on the flats. sence i bought it i have replaced cps with the 5 deg adv all new tune up 3.5 lift 31 tires and less restictive muff. it has tones of power and now millage to boot oh and that was on 85 octane too.

DARKFLY said:

...sence i bought it i have replaced cps with the 5 deg adv...

Click to expand...

What kind of CPS is this? Are you saying it can adv. the timing 5*?

yes you have to buy the high alt cps

The high altitude CPS is available only for the Renix models. On the HO models (1991 and newer) the ECU takes care of timing advance for altitude.