Anyone else noticed that your MPG dips immediately after an oil change, then tops out after about a week, then starts going back down after ~3,000 miles as the oil gets dirty.

I always keep track of my gas mileage, and this has occurred every single time

 

I do observe that the mpg reading is worse when I receive it after the oil change than it was when I handed it over (they ran the engine inefficiently in the shop). I've not noticed that I do worse driving after I get the car back. - Doug

 

I am thinking about maybe switching to synthetic and only changing it every 6,000 or so; instead of every 2,500-3,000 like I do now with regular oil.

 

My transaxle oil studies indicate that as the oil ages, the viscosity decreases which normally reduce the drag. But as 'stuff' accumulates in the oil, it appears to bridge the thin oil lubricating film and drag goes up again:
http://hiwaay.net/~bzwilson/prius/pri_toil_01.jpg

My one test with Mobil 1 0W-20 indicates there was plenty of viscosity and the filter was still effective after 5,000 miles. Based upon that and my transaxle oil studies, I'm adopting a twice a year oil change:

1) Fall -> new filter, 0W-20
2) Spring -> new filter, 5W-30

This means each oil change will go through about 8-9,000 miles but the important part is any accumulated contaminants from the Summer or Winter, will be flushed as each season changes. In the winter, I'll have a lower viscosity oil to help fight the winter MPG blues.

Bob Wilson

 

Bob, I like your charts and your theory makes sense, but I have to agree that Aspie may be onto something. When I got my Prius back from the first service a couple of weeks ago, the mpg reading was definitely worse, on my normal routes, which I attributed it to the dealer lowering my tire pressure, or perhaps changing out the break-in oil.

However, it has since improved most of the way back to normal, and I still have the tires down a bit. I'm starting to think that they reset the computer, so it had to "re-learn" my driving habits. Or they do something that causes it to run less efficiently for awhile.

 

I change my own oil, so I know it's not the mechanic.

I used to get oil changes at Wal Mart, until I noticed one time that they didn't even change the oil filter - no thanks, I can do that bad for free. I do still get my tires rotated there.

 

Watch the tire pressures after any sort of oil change. I use a foreign car specialist who should know better, but he insists on resetting ny tire pressures to the "factory" setting. Worse, he does this when the tires are still warm, it not actually hot.

Grrrrrrr.

 

My manual on my '04 HCH tells me to change the oil every 10k miles. I use 0w20 synthetic (Mobil 1).

The mileage on the first tank is always bad. Why? Because I had to heat up the engine to allow the oil to flower out easier and then run the engine to check for leaks. So, I let it idle for about five minutes total, or about 1-2 mpg.

But I do the change myself, and I know it is done correctly.

 

If you get the oil changed at the dealership, you may want to verify that they are in fact using 0W-20.

0W-20 is not readily available to dealerships in bulk. So, they may in fact be using 5W-20 or 5W-30. This will definitely negatively affect your fuel economy.

From a dealership's perpective, they do not want to carry a bunch of different engine oils as it increases complexity (separate bulk tank for each fluid).

Same for transmission fluid. How many ATFs does a dealership want to carry? Yep - just (1). Not Mercon LV, Dexron VI, Toyota WS, and CVT.

In my humble opinion, just verify that the dealership is putting in the right fluid.

- Suk

 

At the dealership I used for my first (only) oil change, they pulled out 4 individual 0-20 quart bottles of "Honda" labeled oil. And they appeared quite aware of the need for 0-20. I'm sure dealerships vary considerably about this, so good point skoh. BTW, welcome to the club!