I bought new LT All Terrain Tires and my mileage dropped from 14.8 to 11.6, what gives?

Maybe it isn't the tires at all, maybe something went wrong with the truck at the same time which I didn't notice.

Two things I noticed lately are on startup, the truck engine surges from about 650 rpm to about 1200 rpm, after I kick it down it stops and idles normally. ??

The other is, I have heard a louder noise coming from the exhaust which prolly indicates a hole, question is, is there a computer sensor reading the exhaust gasses which may cause excessive fuel use?

 

What tires did you have, and what tires did you change to?

The idle surge at startup is normal.

O2 sensor reads the exhaust. If it has more than 60K miles on it, it probably wouldn't hurt to replace it. (and reset the computer as well.)

 

+1 on the 02 sensor it can **** ur fuel mileage up pretty good.

 

They were standard P265 / 75 R16 for a 99 1500 quad 4x4.

I used the same size but bumped them to a LT instead of a passenger tire so I would have more sidewall stability for cornering and load capacity.

Thanks for the O2 info, will do. But this truck as at least two of them maybe 3 because it's a 5.9 . Also, I don't have a check engine light which normally comes on with a bad sensor.
I had an after thought though, would a hole in the muffler cause a wrong O2 sensor read because there would be less back pressure. idk

 

O2 sensor doesn't care about back pressure, so long as the hole is AFTER the sensor. They won't necessarily set a code either, so long as the PCM is getting SOME info from them, it thinks all is fine, it doesn't know if the sensor is LYING.

You don't need to replace the downstream sensors, all they do is see if the cat is supposedly doing its job. Just replace the pre-cat sensor(s).

The LT tires are also going to be a fair bit heavier than the P tires...... 3 mpg worth? I would like to think not....... but hey, ya just never know.

 

Oddly enough, I just caught an O2 sensor code the other day. I only saw symptoms of impending failure about a half dozen times, and the fuel economy didn't drop much at all. I think I got lucky and bumped into a truck that has the right combination of parts on it.

The hard part now is going to be swapping out the sensors without also swapping out everything from the manifold back. The wife looks nervous...

 

Tell her to do it then.

 

Thank You