Mileage decreased 25 percent - thoughts?

Have a '98 Cherokee Sport w/Up Country suspension 80,000 miles
running 30X9.5 BFG AT's and have a spare on the roof
Getting 14 mpg on the highway as opposed to the std 20-21 mpg
Any ideas to are certainly appreciated - does not seem reasonable that the slightly larger tires coupled with a bit more wind drag on the roof should be so significant in terms of the mileage, however if that's what other folks are seeing I'll leave it alone

thanks

jlmcc2 said:

Have a '98 Cherokee Sport w/Up Country suspension 80,000 miles
running 30X9.5 BFG AT's and have a spare on the roof
Getting 14 mpg on the highway as opposed to the std 20-21 mpg

Click to expand...

Check the oxygen sensors. I've had them go at about 70-80,000 miles on every vehicle I've ever owned equipped with them; the catalytic converter may also be acting up. Poor-quality gas has also made my 2001 go as low as 12mpg. Is power normal or lower than usual?

thanks...

as for power seems pretty normal, although not necessarily as peppy as the 2000 XJ I drive at work

jlmcc2 said:

Have a '98 Cherokee Sport w/Up Country suspension 80,000 miles
running 30X9.5 BFG AT's and have a spare on the roof
Getting 14 mpg on the highway as opposed to the std 20-21 mpg
Any ideas to are certainly appreciated - does not seem reasonable that the slightly larger tires coupled with a bit more wind drag on the roof should be so significant in terms of the mileage, however if that's what other folks are seeing I'll leave it alone

thanks

Click to expand...

OK try this....

Pull the rack and spare off the roof, put stock tires back on, that should do it.


It is very reasonable to expect a loss in milage for both those mods. Did you change gears to compansate for the bigger rubber? If not that can be a biggie right there.


Rev

Did it just start all of a sudden ? Our fleet started running like crap this week, the XJ, both TJ's and the oldsmobile. I'm expecting a decrease in milage when I fill up again on friday...they're doing something with the gas in Pa again, happend this past winter too for about a month when prices shot up.

"does not seem reasonable that the slightly larger tires coupled with a bit more wind drag on the roof should be so significant in terms of the mileage"

Seems pretty reasonable to me.

My first impulse it to think you have a dead oxygen sensor but being an OBD-II vehicle it should tell you if the oxygen sensor is bad.

I wish you would of give us more information on the vehicle like:
* How long have you been running the 30 inch tires
* Did you change the speedo gear when you went to the different size tires

If the larger tires are something new then changing the speedo gear needs to be accomplished. If you are unsure about the speedo gear then take a drive on your local interstate highway. Reset your trip odometer then drive 20 or 25 miles and stop at that mile marker - start a mile marker 32 stop at 52 or 57 for example.

Compare the odometer reading to the distance you just traveled. Lets say you traveled 20 miles but your odometer says 18.0 miles. By doing some math you register only 90 miles when you actually travel 100 miles. That can throw off your gas mileage. Do not use a GPS to check your speedometer and think things are just fine. My XJ is off 0.6 miles per 100 miles travel BUT my speedo registers 4.5 % slower than actual. When I drive at 65 mph I am actually going 68 mph.

If the fuel mileage loss is an all of a sudden thing then I'd consider oxygen sensor first. I had my oxygen sensor fail on my 86 mustang 5.0 and it went from 20 to 16 mph. With a tank I'd fill up every 12 gallons I was not getting my normal 240 miles/tank. The vehicle will run fine, it just running rich and you will not notice that around town or on the highway, just the highway.

Also the tire on the roof can be a big cause of your loss of MPG. You just increased the frontal area of the XJ and increased the coeficient of drag. Get that tire off the roof and back on the stock holder inside.

jlmcc2 said:

Have a '98 Cherokee Sport w/Up Country suspension 80,000 miles
running 30X9.5 BFG AT's and have a spare on the roof
Getting 14 mpg on the highway as opposed to the std 20-21 mpg
Any ideas to are certainly appreciated - does not seem reasonable that the slightly larger tires coupled with a bit more wind drag on the roof should be so significant in terms of the mileage, however if that's what other folks are seeing I'll leave it alone

thanks

Click to expand...

I've just returned from a thousand mile or so round trip to Maine from Long Island. 235 x15's, with new spark plugs - the overall average is 19.7 mpg. With the stock gearing and auto trans, my average speed was kept under 70 mph, probably 60-65 overall. I thought I was going to do better considering it's a pretty flat route. I only have 40K miles on the vehicle. Is your auto or stick and at your cruising speed, what's your RPM's?

martin said:

If the fuel mileage loss is an all of a sudden thing then I'd consider oxygen sensor first.

Click to expand...

In addition to this, check your high pressure fuel line that plugs into the fuel rail toward the front of the motor. They've been known to spring pinhole size leaks at the formed plastic section of the line. It's only detectable when the engine is running and will spray a fine mist of gas on or around hot engine components---can lead to [external] combustion if goes undetected.

This obviously will cause a sudden loss in fuel mileage.

Good luck.

My own experience has been that when I make a change to the Jeep the gas mileage gets really bad and then gradually gets better again. Maybe the computer is using the wrong values and it takes a while to build the new data or something. Don't forget to check and make sure your speedometer is accurate now too.

looks like the first step is to check the spedometer - will do (I did not modify it when I installed the 30's.

I have had the spare inside on a few occaisions in the past - did not notice any significant change (does not fit on the internal mount now)

Trans is an auto

thanks