The miles per gallon on my 2009 Toyota has suddenly decreased by about 3 mpg. Anyone know why?

oil different type or weight? synthetic is said to increase MPG? some imports are fussy about brand of plugs.

I did use a different oil on my last change....went to a "high mileage" type. I can easily try going back to my old standby and maybe I'll ask them to change the wires along with the plugs next time? Thanks!

During the winter months , gas stations use oxygenated fuel which will reduce your gas mileage. Colder winter temps and worse road conditions will do the same.

Be sure to use TOYOTA plugs or the EXACT same plug that Toyota used.. Using Generic Autolite/AC Delco/Champion/Motorcraft or other generic brand plug WILL get you almost exactly 3 MPG LESS gas mileage than the OEM NGK/ND brand spark plug. Many shops use the cheaper shit, as most customers are not willing to pay the difference... Typically NGK/ND plugs are $8 to 15.00 each. Generic ones are $2 to $4.00 each.

Your car, almost the same as my 4-dr sedan wants/requires NGK Titanium tipped plugs, same as what was originally installed in your car when new. I, too, have gotten >40mpg from day to the present; my car has 108,7xx miles on odometer. To date, I've never even looked at the plugs, my engine idles smooth, no hiccups/misses/rough idle. I'd suggest cleaning the throat of your throttle body, no need to removed it from the intake manifold, The Car Care Nut has a youtube video showing how to clean the throat of the throttle body on Toyotas. I keep 35# tire pressures on my 15" rims; check to be sure the tire pressures are at least 32# on each tire. You might want to consider replacing the Mass Air Flow (MAS) sensor. DON't try cleaning the existing one.