poor gas mileage on 2012 tundra

I recently had my catalytic converter replaced due to engine light. I don't recall the specific error code but it indicated catalytic converter and o2 sensor problem. Once it was replaced I noticed poor mileage and after a couple months I took it back to the mechanic and they replaced the catalytic converter indicating that was the likely cause. Well I have recently had the 120k checkup completed. It has only been about a month since the converter was replaced and 2 weeks since the 120k checkup but my gas mileage is still pretty atrocious. What else can I do to try and determine why my mileage is not improving? I used to average around 300mpg on a tank of gas driving around the city now I'm getting maybe 250.

 

I just did a brake job on brothers 4-Runner and he had a frozen piston on one his calipers. He said after I replaced the calipers, his mileage increased a few mpg’s

 

I don't think that should be the case as I also had my brakes checked while it was last in the shop. thanks for the input.

 

Let me take a guess. Is it a Flex Fuel vehicle?

 

no, it's a 2012 tundra sr5 no flex fuel.