So here I am, perusing various posts, and again I see people with tires much bigger than mine with 4.56+ gears, making up to 20 MPG. (Or so they say.) But almost everyone says that the 13 MPG I'm making now is horrible and that something must be wrong. (It actually went up 1 MPG when I installed the Magnaflow muffler.)

I've already changed the upstream O2 sensor, but I was wondering if I'll see an improvement by changing the downstream O2 sensor. I know that '95 & earlier XJ's, the downstream sensor only monitored the cat. However, there's been some indication that '96 & later (OBDII) XJs actually use the downstream sensor in fuel calculations.

Can anyone confirm or deny this?

PS. I know this is more complicated than that. I have a 62mm TB, 99+ intake manifold, upgraded fuel injectors, etc. and I know they all factor in. But I'm just wondering if the downstream O2 sensor SPECIFICALLY affects mileage in MY year Jeep. Thanks.

 

Yes it does. The rear O2 does contribute to fuel trim.
13 MPG highway? City? Mixed? I'd say that's about right for city driving.

 

Thanks. From what I read, I thought it might, but couldn't be sure. OBDII was key. Anyway, 13 MPG mixed, but more city street driving with short stints on the highway. I'll go ahead and throw down the $$$ to get a new downstream O2 sensor because I think it's still original and I changed the upstream O2 sensor last year (or two ago). I appreciate your input. I'll write back here when I change it and do a comparative test.

 

I don't think your mileage is all that far off for mostly city driving. My last tank of gas was 100% city driving, I calculated 12.5 MPG. Not exactly spectacular, lol. They weren't rated much more than that by the EPA when new.
20-ish on the hwy? Sure. The older Renix system was easier on gas too I think.

 

When I replaced mine, it ran better/smoother so I did get better mpg, but I knew to replace it cause it threw a code at me. I don't think replacing it improved the mileage, just brought it back to where it should have been...

 

Thank you for asking that question!! I have the same setup that you posted and have been researching what you asked. I just ordered an up and down stream from Rock Auto. Ordered the NKT brand, seems to be highly recommended as they are OEM from what I have read. $72 shipped for the both of them which is a kick a$$ price!!

 

Thanks for mentioning Rock Auto. I was considering going to the dealer, as I've been burned by aftermarket sensors before. The way I figure, it's not throwing a code, but I think it's the original O2 sensor. I have 205k on the vehicle and changing the sensor out probably isn't a bad idea. Plus, I've changed out just about everything else, so I might as well.

 

Luckily NTK is the OEM for O2 sensors in all the Chrysler Jeeps. No reason to spend $150 at the dealer (I'm guessing, probably more...) when they're $30 for the exact same part!

Anyway mister OP 13 MPG sounds a wee bit low if you're able to get 20 on the highway. You driving with a lead foot?

 

My bad, I fat fingered and meant to put NTK.

 

Lead foot? Nah, I'm even scared to push it to 75. It's not exactly stable like a sports car at that speed!

One thing: I have yet to install new speedo gears. Someone said to multiply my odometer mileage by 1.1 to make up for that. When I did, my mileage came out to 15 mpg. I believe that's book right there...15 city / 18 highway.