Fuel Efficiency Issues

[GerryL98]

I got new tires, 100K service (non dealer), transmission fluid service (non dealer) and I'm still experiencing a dramatic drop in fuel mileage that started almost three months ago. I'm also hearing a little bittle of buzz/rattle from underneath. I brought it to the transmission shop and they only heard a slight noise. They said the transmission is fine but I should get a full service shop to have the catalytic converters checked out. This could be the cause of the noise and the fuel mileage drop. Wouldn't a bad cat show up in my smog inspection which I passed with flying colors three weeks ago? The shop I was referred to is busy and can't take my car until Wednesday so I have a little time to research and get second opinions. BTW, it's a 2005 Silverado LT Crew Cab. thanks

[becker87]

Did you change tire size by chance?

[GerryL98]

Did you change tire size by chance?

No. I brought it in to a shop with very positive Yelp ratings. Tech said not to worry about the cat noise until it triggers the check engine light. He ran some diagnostics and said the fuel trim was high at +15. He said the mass air flow sensor and the K&N air filter were both extremely dirty. He did a cleaning of the sensor, recommended a stock air filter, performed a fuel induction service and cleaned the throttle body. After service the fuel trim was +6. We'll see how it goes after a couple of tanks of gas.