I just got a set of new tires. Went from stock 225/50/18s to 245/45. There is a noticeable drop in mpg. Is my mind playing tricks on me? Anyone with experience the same? I am assuming you guys with 20s are getting less mpgs.

 

If you went from an all season run of the mill tyre to a high performance summer only tire, there would be some difference. How noticeable? I image a few points.

Also, 245/45's rotate slightly more revolutions per mile than 225/50. According to one tire calculator, 5 revolutions more, so negligible.

Wheel diameter has no influence so long as outer tire circumference remains the same as stock.

Are you manually computing your mileage or relying on the dash readout?

 

Some tires do have more rolling resistance than others. Your mind likely is not playing tricks on you.

 

Did you change the rims? Your remark about 20s allows some vagueness to creep in.

Just talking tire size doesn't allow very accurate speculation. The more information we have as to what we have for variables and constants helps a lot.
-wear on old tires
-rim size
-full tire spec
-tread depth on new tires
-calculation method
-no oil change?
-do you always use auto mode on your AC?
-etc

 

wheel color makes a difference too. Something about heat absorption from the sun. Jk

 

Maybe the new tires weigh more than the old ones, most of the tire sites have weight specs to compare. Rotational mass / unsprung weight makes a difference in performance and may also affect gas mileage if they are heavier.