Changed my oil and MPG went from 53 down to 46. Wth??

how are you calculating mpg's before and after?

 

Your observation is after how many miles traveled since the service?

 

Try lower the oil level and be sure it is not all the way at the full line.
Also try tire pressure Front 42 and Back 40

 

i think you need a fresh tank and see what the long term is. pretty tough to judge by driving around and watching the screen.

 

Give it a day, Wth

 

some say it can, but my guy always puts in what the manual says. it's always a quarter inch over the full mark and i always get the same 55-65 mpg.

 

From my overall experience with gasoline engines, the best oil level is halfway the Empty and Middle marks.

 

I usually try to fill just a shade under the full dot (1/8" to 1/4" under), as personally I like it to be at least somewhere near full.

When the Toyota dealer was doing the servicing however then, like bisco, it was routinely 1/8" to 1/4" inch over the dot.

There may be a really small fuel efficiency benefit to running the oil somewhat under the full dot, but as far as I can tell it's only small. I've been getting 53 to 55 MPG the whole time.

There does however seem to be a point, that if you overfill past, then there really is a noticeable deterioration in FE. I've never been there, but from what others have reported here it seems to start somewhere between 1/4" and 1/2" over the full dot. Remember that if you overfill then you can get frothing of the oil, giving rise to faster initial oil consumption and excess oil drawn into the intake (which can cause other ongoing problems and reduce FE).

 

Just a shot in the dark here are you sure the oil was 5w3o and not 20w50 or other heavy oil, probably not but worth checking.

 

I never noticed this, and I changed the oil on our 2006 Prius for the 16th time yesterday (16th time on that car, not all in the same day =). It's nice to hear about new findings from new members.

 

Why in the world would the trip odometers have anything to do with computed mileage????

 

^ It doesn't

^ This.

 

Uart is correct. The odometer cannot possibly be the problem.

However this should be a reminder to everyone NOT to over fill their engine with oil.

I my oil was 25% over the full line and my mpgs were around 40. I had it dropped to just under full when I put my summer tires back on and I am now at 55 mpg after around 70 miles.

Granted a large part of the increase was probably found by going back to summer tires but Im sure the oil was part of that. 15 mpg increase is pretty high for it to be just tires.

 

I agree with this, it causes additional crank windage. .