2002 Volvo S80 gasoline mileage

When I first bought my used Volvo I would fill up the gas tank with 93 Octane and the electronic gasoline meter would say I can get 585 miles to the tank but in reality with city driving I would get about 425 miles. Then I changed the spark plugs to the Bosch Iridium which cost almost 9dollars a plug and the oil & filter and the air filters and now when I fill it up with 93 Octane it reads 460 miles to the tank and I get much less than the original 400 miles on city streets. I always went to the same gas station and received a reading of 585 but when that changed I went to 2 other gas stations and it remained at 460 miles.

Could it be the spark plugs? Should I stick with original Volvo plugs. I looked at the old plugs I removed and they were really old and worn and had no name branded on them.

Thank you.