Exactly the same thing happened to my 2004 Camry (non-hybrid). After switching to full synthetic oil with the same viscosity, the mpg dropped about 20% (from 30 mpg to 24 mpg, highway, manually calculated by odometer reading and gas receipt, no driving pattern changed). In my case, in addition to the oil change, old engine air filter was replaced to new one. MPG is still low after more than seven times fueling of a full-tank. What can I do to get the old good MPG?

 

24mpg is what it should get city mileage non hybrid 4cyl model.. and 32 is what they say you should get highway...i got 29mph at 90mph...which is still pretty good...

I run synthetic as well but if anything it should help your gas mileage not hurt it by any means.. Your engine could still be getting broke in...Normal city driving i get around 24-27mpg on good days.. I think you guys are being over dramatic about the mileage results your seeing. If its within specs of the dealer you have no worries...Gas has alot of factors...stop and go traffic...ac on or off...temp outside etc..

 

My driving pattern is really simple. 150 mile highway on Monday and Friday. So I can calculate my MPG every weak and was constantly ~30 MPG for a year, but the next week after oil change + air filter change + valve sterm change (rear passenger tire) MPG suddenly dropped to ~24 MPG.

I'm still doing some googling about this really sudden 20% MPG drop. I found that wrong type of engine oil could be a reason of this symptom for hybrid cars, for example normal 5W30 instead of 0W30 for hybrid, but it's not probable for non-hybrid cars. It seems that some professional inspection is needed.

 

24mpg is within specs ....24-34 mpg is what the dealer stickers says i believe cant remember any more..
Remember that the highway specs are rated at 55mph not 70 mph as are most highways now days. The highway mileage is based on this rating.

I do find it kind of strange that it would drop but like i said there are tons of factors that can make or break your gas mileage. Outside temps, cruise control, driving habits, tire pressure etc. If your car is still new lets say under 20k its still getting broken in. Til this is done its really hard to tell if you have problems. I have 120k on my 05 i get 24city and about 29 highway. Ive only got 30mpg doing 70mph on the highway but i had 18wheelers passing me, so to pump out and extra 20mph for 1mpg is fine by me.

Even and oil change and new air filter these things shouldnt decrease your mileage by anymeans it should increase it so i see your problem. However its within specs so there should be no worries.

Gas mileage should be factored in per tank, im not saying your calculations are wrong but probably not very accurate as if you were to do it per tank. Also another factor would be how much gas you used to how much gas you put back in...Sometimes you might stop it at the 1st click sometimes you might round it off or sometimes you fill her up till she poor out. This also could throw it off and give you a false reading . I always fill it up to the neck this way i know that i put the same amount of gas each time, i use it more as a visual reference . However i dont recommend filling it past the 1st click cause it may cause damage.

Or just maybe your engine is getting use to the synthetic oils and its properties and adjusting itself. Cars now days are all computerized and the ecu is always making adjustments that fit the drivers needs and adapting to them.