I have a 2006 Toyota Prius bought used exactly two years ago with 45K miles. Since then I have put another 38K miles on it. It started off by giving me around 48-51 MPG depending upon my driving conditions. Earlier in the summer I realized the mpg started gradually going down. I changed the air filter and it seemed to make some difference. I changed the tires and it did not make any difference. The mpg I was getting was between 36-38 and upon reading some more, I recently changed my 12V battery to the Optima one. I now average 40 mpg which is nowhere near 48 mpg which I was getting before. I change oil every recommended interval so I don't really know the reason behind this drop. My tire pressure is fine, I just got wheel alignment done, it has new air filter, even a new cabin filter, etc. My driving habits haven't changed. One more thing is I installed the new battery just this weekend and my 40 mpg measurements are after 175 miles in. Any idea why the mpg dropped? Thanks. Oh, I'm not a troll. Misinformed maybe. LOL!
check the engine oil to make sure it isn't overfilled. - that will affect mpg. The O2 sensors getting old will lower MPG, and the automatic transmission fluid and engine coolant should be changed when possible.
Cycle the emergency brake to make sure the cable isn't sticking, clean the Maf filter, and yes, the oil gets filled 4/5ths, not full, it actually makes a difference, reset the trip meter after unplugging the 12v for ten minutes.
OP, you didn't mention anything about checking the tire pressures. If they are less than the door placard (35 Front and 33 Rear) this will affect MPG. Running them 3-4 PSI higher than the placard often helps to get even wear across the whole contact patch and has the additional benefit of increasing fuel economy a bit too. Hope that helps.
welcome! jack up the car and make sure all the wheels spin freely. might need a maf/throttle body cleaning, maybe an injector issue. if none of the above help, could be that your hybrid battery is going south. it's a fairly complicated test, unless you're a good diy'er. all the best!
