I have a 95 vert with 35k on the clock. I have performed some minor mods that include going to 315/35 's on the rear and 285/40 's on the front. K & N filter and Magnaflow mufflers only.

I took the car on a small trip (about 220 miles) this weekend and could not help but notice that my gas mileage had dropped to about 22 mpg on the highway. This is down from about 25.5 that I am accustomed to getting on interstate driving.

About two weeks ago, my Service Engine Soon light came on for about 3 minutes and went back off. It hasn't came back on since.

Could this be an oxygen sensor problem or a result of the mods, mainly the wider rear tires?

 

Tires, nor any of your other mods are possible causes. There should be a historic code set, so just take it to Advance or whoever in your area offers a free scan.

 

i went thru the same thing about a week ago, replaced bad o2 sensor everything is back to normal. pull your codes with a paper clip then, look in your shop manual for description. Ill bet you'll see a code for a bad o2.