98 jeep gas mileage

[r5active]

Is there a way to increase gass mileage on a 98 jeep cherokee 4.0 6cyc. I was thinking of increasing the tire size. Has anyone tried that? will it work? thanks ken

[Ray H]

Originally Posted by r5active

Is there a way to increase gass mileage on a 98 jeep cherokee 4.0 6cyc. I was thinking of increasing the tire size. Has anyone tried that? will it work? thanks ken

I dont think you will see much, if any MPG difference by going up a tire size or two. Its not really the RPMs that determine your MPG as much as throttle position. The more air you let into the engine, by pushing the accelerator, the more fuel thats going into the engine. Larger tires may actually hurt your mileage under some condtions because it takes more throttle to get them rolling. Once on flat interstate, the big tires will probably help, but around town or on hills, they will probably hurt.
There are alot of aftermarket intake and exhaust pieces that are supposed to increase mileage. I dont personally buy into them. They may help a little but by the time you spend several hundred for them, you are are the hole to save fuel. The best way to save fuel is to make sure your Jeep is in good operating order (ie: tuneup, tire pressure, lube, etc) and modify your driving habits by slowing down, timing lights, slow takeoffs, etc.
I know thats not the answer people like to hear, which baffles me because its FREE. It seems most people would sooner spend $300 to get 2 MPG more than slow down a little for free and get 2 MPG more. It just doesnt make sense to me . Anyway, thats my take on it.

EDIT: Oh,yea, Ive got 30" tires on my XJ with stock gearing. No noticable change in average MPG. I did notice a small power derease though.

[sector95]

Ken,

A good old fashioned tune-up would be a good start. Have an upper intake track cleaning done (this typically covers the injectors and the intake manifold/throttle body). Every oil change (3000 miles) pour a bottle of Chevron's Techron in the tank.... this stuff is expensive ($9) but it works; IMHO, the other injector cleaners are a waste of money. Make sure your crankcase vent system is working properly. The engine has no PCV valve; it uses a sort of permanent valve (CCV) mounted in the rear grommet on the valve cover. Fresh air is drawn in from the airbox and through a fitting on the front of the valve cover. With the engine running, remove the small, hard plastic hose from the airbox that connects to the valve cover; you should feel a good suction. If not, your CCV is plugged; remove the hose and squirt some intake or carb cleaner into the elbow. Reconnect and check for suction at the front hose. If it's really plugged, you'll need to remove the valve cover to clean it more thoroughly. An operating system relieves blow-by pressure build up in the crankcase so the engine isn't fighting itself by working against a pressurized crankcase.


IMHO, aftermarket intakes are a rip off..... the major restriction to the intake airflow is the more than 90deg bend above the throttle body....I've not seen any aftermarket kits alleviate this because there's very little clearance between the hood and the throttle body. They look cool and they are noisy (intake roar) but the "seat of the pants" dyno says you a'int gettin' much else for your 300 smackers.

Larger tires will probably result in less mileage. Try putting a couple of pounds more air pressure than factory specs in your tires (but never above the sidewall rating on the tire)...you'll sacrifice a little ride quality for a little less rolling resistance. CHECK THE TIRE PRESSURES REGULARLY (I see more cars running on under inflated tires....some for so long the tire has worn because of it...this is dumb and dangerous and it wastes fuel). Rather than larger tires, buy tires that are designed more towards highway driving than off road use (if that's how you primarily drive your Jeep). Those gnarly off road gumbo mudders have a much higher rolling resistance on the pavement than street or dual purpose tires.

Learn to drive smart. Learn and get a feel for how the auto trans shifts and try to get it into converter lockup/OverDrive as quickly as possible (OD/lockup won't engage on the AW4 until approx 42mph...but once engaged will remain engaged at slower speeds unless the brakes are applied). I find I can get good acceleration and quickly reach lockup/OD by running the engine at about 2000 rpm. Anticipate red lights..... try and slow early so you don't have to stop. Drive smoothly; no jack rabbit starts and practice maintaining your speed.

After all is said and done, these techniques garner me about 18-20mpg on my '00 XJ (4.0L; 4WD & AW4 auto) in mixed local & hiway driving. In short, the XJ is no mileage champ.

Good luck.....

mike
00XJ