fuel mileage has dropped

[tarmac2dirt]

Well it seems to have dropped about 4mpg from what it used to be. Ran a trip of about 120 miles and refilled the tank and calculated at

13.8 mpg. does this seem low to you guys. This is a stock 1988 MJ long bed 2wd 4.0 engine with 706 injectors and a 60mm throttle body cruised at 65 with a/c on one way and off coming back home. Tires are a little larger 30x9.5-15. I kinda thinking the O2 sensor is bad, happened once befor about 10 years ago. change the O2 and got my mileage back. Does it sound like the sensor has gone bad again. probably has about 60k since the last sensor change. Is that about limits for an o2 sensor 60k miles. I'm going to go ahead and chane it but is the any other things killing the mileage, MJ is at 200k on the clock so it is getting tired too. thanks Paul

[pizzaman09]

Are you calculating MPG based on the odometer or off of say a GPS distance?  Since you have larger tires the odometer distance will be less unless you changed the speedometer gears.  That would account for about 1 mpg.

 

13.8mpg does sound low to me, I'd expect 18-19mpg.  Spark plugs and O2 are likely decent places to start looking.  Also checking to see if your manifold bolts are tight.  Do the basics of a tune up.

[Tex06]

That's much lower than I get with 4x4, 31" tires, and pulling a load. 

 

Check Fuelly.com for comparable but I'd definitely test the O2 sensor, inspect the spark plugs, have you ever changed the distributor and spark plug wires? 

 

13 is very low unless you're pushing 80+ mph everywhere you go or just putting around town with no interstate time.