I have a 91 B-2600i 4x4 with 90K miles. My gas milage has been getting worse and is now down to 14-15 MPG. I drive mostly in town and have 31" tires but I do not think that it should be that bad. I read a few posts that said to check the 02 sensor, but I don't know how to do this and don't want to spend $60 to replace one if it doesn't need it. Any advice would be appreciated on the sensor and what else to check to improve my gas mileage. Thanks!
-Rob