Not the tires..
Unless you drove 2 tankfuls since last weekend when you installed the tires..
Mileage on my XJ has dropped with the colder weather we've had in the last month or so, by 30-40 miles per tank.
The copper plugs shouldn't have hurt you, IMO, they are the best out there for most engines.
You might want to play with the gap size.
Don't still have the old plugs, do you? Could always swap them and check..