Hi All, I have been trying to figure out why my MPG has dropped on both of my Prius's and I have read posts that suggest over filling the oil in the crankcase will cause mileage to drop. Is this a myth or is there any truth to this? If so, exactly how much over the full mark would the oil have to be to affect the miles per gallon? In both Prius's the oil is over the full mark by approximately 1/8 of an inch. All comments are appreciated. Thanks all.
It's commonly known among the members here that overfilling the Prius results in a measurable drop in mileage (perhaps 3 or 4 mpg). I have never tested this myself and I make sure when I change my oil that I'm just under (perhaps 1/4") the full mark. I can certainly see how the foaming of oil from being overfilled can reduce fuel economy even though I have nothing official to cite that supports my theory. In addition overfilling the engine can cause damage to the engine itself.... If you're over 1/8" an inch your unlikely to have an significant problems....but I'd say you may get less fuel economy as a result. Mike
First hand I can tell you that too much oil in the crankcase WILL drop your MPG. I now have the dealer put in only 3.5 quarts which leaves the level just below the full mark. Didn't you post a similar question in another section of the board?
Yes, I wasn't sure which area was the best one. One question: I also use full synthetic oil in both Prius's. I know it's slippery and not prone to breaking down @ high temps. Does it make any differance on the MPG?
Just to be contrary, I put 4 litres of oil in Pearl and didn't notice any decrease in mileage. The level was 1/8" over the full mark. I put in Mobil 1 and didn't notice any improvement in mileage. Yes, the next change I only put in 3.5 litres of Mobil 1 (level just below the full mark) and again noticed no change in mileage. BTW, switching to Mobil 1 did effect the anti-Prius (2001 Nissan Pathfinder SUV), improving its' mileage enough to pay the extra cost of the oil 3X. And to put up one more point, one of the litres of oil I put in is 10W30 while the rest were 5W30 (using up the oil I have in stock for the anti-Prius). I -suspect- some of the mileage effects are imagined. Note that Bob's car is a Prius Classic, and his measurements are probably more accurate than mine (I keep a written record of fillups).
My car was delivered, as most seem to be, with oil 1/8 in. over from the factory. I have had it right on full and 1/8 over with no differences in anything. Nothing is foaming or harmed. I am making a guestimate that 1/8 in over is about 2 oz of oil. That is insignificant. A quart or more over may be a different story. Of course there is a difference in how long the oil drains down on the oil level check. With less than four qts. including filter, I would prefer the oil to be on full rather than under. There are probably less than three qts. in the pan while running, not much reserve. I would consider all the other mpg factors first.
I noticed lowered mileage, dropping from 49MPG tanks to 46, early on after a dealership oil change. Soon after, I either drained some oil or did my own oil change (I can't remember which), keeping the level a bit below the full line. Mileage improved. Not a scientific study, but it convinced me to always slightly underfill. I've been running with oil 1/4" below full for the past 4 years. 126K miles and no problems yet. I use Mobil-1.
My understanding is that the lubricant in the car is replaced by the dealers after the car is shipped to them. So the overfill situation is once again due to the dealers treating the Prius as, "just another 4-cylinder Toyota." That leads me to wonder how many brand new Prii leave the dealerships with 10W30 in them.
The original oil fill is from the factory in Japan. The dealer does not change it. Why would they? It is a made up rumor. It is easy to tell, as the original oil filter is Denso, but much different contruction than the Denso sold at dealers. There is Japanese air in the tires, and Japanese water in the coolant as well. Toyota japan overfills the sump by 1/8 in. as it makes absolutely no difference except to those who think the dipstick is a micrometer. It corresponds to their 3.9 US qt fill spec. which they haven't changed in all these years. I use about 2-3 oz less on my change and I am right on full. I drain my oil for about an hour, and wait that long or more to check the level. After running it. They should just recalibrate the dipstick to their recommended oil amount. Then all this obsessing about 1/8 inches could end.
