Why is my truck using so much gas. Runs good but has suddenly become a gas guzlr on 2000 Chevrolet Silverado 1500

Warm starts difficult. have to push down on peddle to start. I replaced the throttle censer. New gas filter, new air filter. New gas pump. Really uses a lot of gas.

Check fuel pressure and long-term fuel trim.Post specific code #s if available.
1 more answer , 2 more comments
check for codes
I did and it said the rear banks were not normal even after changing the O2 censors. I assume this means the "Y" pipe needs to be changed.
the converter could be plugging up