so i just went through my second tank of gas since ive had the xj. i averaged 14-15 highway. why? in the past month its had a new cat, new plugs/wires, air filter, oil filter, exhaust, cleaned the tb/iac. i threw some 245 mtrs on it, but i didnt think it take that big of a hit. i have yet to correct my speedo. but i doubt that has anything to do with it. i want to replace the cap and rotor on it. what else should i start looking at?

 

The spedometer could definately have something to do with it as you're bigger tires are traveling further with each revolution than your car is registering. Combine that with the fact the tires will hurt your economy slightly, and you have the end result. BTW, with my stock Cherokee and stock size tires I'm only getting around 17ish on the highway.

 

did you check your o2 sensors? the guys at JP mag picked up about 10 mpgs after replacing a crap o2 sensor. but they started around 6 mpgs.

 

The new tire size makes a bigger difference then you think.

 

the speedomiter also tell you miles, if your speedometer aint right them you caculations are gonna be wrong

 

X2
But with my 100% stock XJ im geting 18MPG mainly HW but its also winter.

 

My Buick Le Sabre just started getting worse mileage as well, so I looked into it.

First, it may be the temp - colder temps generally equal worse mileage.

Second, I asked the gas station where I filled up, and sure enough, they overhauled their fuel pumps last week.

You see, as fuel pumps wear, they deliver more gas than new and than indicated. I have a friend who makes a great living overhauling fuel pumps in his shop. Every once in awhile, I'll check to see who has overhauled pumps, and quit buying gas there.