I know this question has been asked a million times but here it goes. I have a 98 accord I4. My mpg's for the past 4 years have been roughly 28-31 highway and 24-26 city. About two months ago they changed to being 23-24 highway and 19-21 city. This happened almost in a blink of an eye. Here is what I have checked and or replaced.
spark plugs brand new ngk iridiums, plug wires, cap and rotor brand new.
Air filter has less than 5k on it. Tire pressure is 35 all around with no balance or allignment issues. I have run seafoam through the system along with techron injector cleaner multiple times in the past. I replaced both 02 sensors after getting a cel. The only thing I am suspicious about involves my rims. I went froms stock 15's to 18s HOWEVER my total diameter is the same because i got lower profile tires. The stock 15's were very thick rims and weighed as much if not more than these new ones so I doubt that weight is a problem. I have run out of ideas. I drive the car the same and just as a test i drove it like a grandma staying under 3k rpms for an entire tank with the same 21 mpg result. I have heard of this "winter blend" of gas yielding less mpg but in 5 years of owning this car the mpg's have never even come close to what they are now. Any ideas on what could be the problem? Thanks in advance