Oil weight observation and fuel economy

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Dec 27, 2019
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351
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Nevada
2018 Mercedes GLC.

Have been running Pennzoil Platinum Euro L SAE 5W-30 for the last three changes or 14k miles. Average fuel economy stayed at 25.5-25.7 mpg during this time. I recently changed to Valvoline European Vehicle 5W-40 and noticed my fuel mileage has dropped to 24.7 mpg.

I thought that was interesting.
 
2018 Mercedes GLC.

Have been running Pennzoil Platinum Euro L SAE 5W-30 for the last three changes or 14k miles. Average fuel economy stayed at 25.5-25.7 mpg during this time. I recently changed to Valvoline European Vehicle 5W-40 and noticed my fuel mileage has dropped to 24.7 mpg.

I thought that was interesting.
Lucky you only lost 1 mpg.
 
How are you arriving at your calculations?

Ed
The vehicle's dash. Nothing hand calculated. Just found it interesting it stayed the same as long as I had 5w-30 then changed once 5w-40 was introduced. I'm pretty sure it will trend down further before it settles based on the trip averages I have seen.
 
As is always the case in these relatively frequent threads on oil grade and fuel economy, it's not that there isn't an effect but it's deep into the noise of everyday driving. Observing something is the easy part but then being able to ascribe the observation to one variable among many minor ones is a far different story. There's no way anyone on here will ever be able to do that. For one thing the energy content of gasoline (apart from winter/summer blend changes) can be up to 4% even at the same gas station.

Here is an excellent post listing many of the other significant variables in daily driving. It's about gasoline but the items he lists are applicable here as well:


And another one specific to oil grade:

 
Crazy as it sounds I've noticed a difference moving from HTHS under/over 3.5 My current fill is 3.6 and I can feel a difference in how the engine is turning while on the highway.

It reminds me of the experience In had with the factory fill.
 
Crazy as it sounds I've noticed a difference moving from HTHS under 3.5 to over 3.5 My current fill is 3.6 and I can tell a difference in how the engine is turning while on the highway.

in-your-head-dolores-oriordan.gif
 
My mileage didn't change when I went from 0w-20 to 0w30. But, when I went from Pirelli Run-Flats to Goodyear Exhilarates, I dropped 1-1.5 mpg. This is hand-calculated over 17,000 miles with the Pirellis and over the past 10,000 plus on the Goodyear tires. Of course, gas pumps may not be accurate, and technically there could be a malfunction in the fuel/combustion system that caused it, but, it was a sudden drop that coincided with the tire change, and the drop has persisted and stayed consistent. The Goodyears are MUCH "stickier" in hard turns and the car is able to hold it's line much better, unlike the "skating feeling" of the apparently much harder Pirelli compound. I'll take the drop in MPG for the better cornering.
 
During the course of an oil change interval in my Nissan Versa I sometimes see variations of 4-5 MPG from one tank to the next and 1-2 MPG variation isn't at all uncommon. Too many things effect fuel mileage to pin it on the oil in the crankcase. I usually use 15-20 year old 10w40 conventional but the last OC I used 10w30 PP synthetic, if anything my overall mileage on this OCI has decreased on the 10w30 synthetic. I don't blame it on the oil but a combination of variables (traffic conditions, wet/dry roads, trip distances, road terrain, etc., etc., etc.)
 
KV100 Euro L 5W-30: 12.1
KV100 Valvoline 5W-40: 13.5

Your drop in fuel economy is from something else.
That was my thought as well but the only thing that changed was the new fill.
As is always the case in these relatively frequent threads on oil grade and fuel economy, it's not that there isn't an effect but it's deep into the noise of everyday driving. Observing something is the easy part but then being able to ascribe the observation to one variable among many minor ones is a far different story. There's no way anyone on here will ever be able to do that. For one thing the energy content of gasoline (apart from winter/summer blend changes) can be up to 4% even at the same gas station.

Here is an excellent post listing many of the other significant variables in daily driving. It's about gasoline but the items he lists are applicable here as well:


And another one specific to oil grade:

I was thinking fuel as well but that would have happened last summer as well and did not with the 5w-30.
Automotive OEMs would give their eye teeth and first born if they could directly attribute a single step change in viscosity grade to ~4% change in fuel efficiency.

I don't doubt that you experienced this change, however, you can't reasonably state that it was the viscosity grade difference that caused it.
I did state an observation, not a cause.
I don't think you can judge on 1 tank.
I run m1 0w-40 in my Mazda, don't see a mileage change.
The decrease has been steady over the last 800 miles or so, I wish that was one tank. It now is at 24.5 from 24.7.
My mileage didn't change when I went from 0w-20 to 0w30. But, when I went from Pirelli Run-Flats to Goodyear Exhilarates, I dropped 1-1.5 mpg. This is hand-calculated over 17,000 miles with the Pirellis and over the past 10,000 plus on the Goodyear tires. Of course, gas pumps may not be accurate, and technically there could be a malfunction in the fuel/combustion system that caused it, but, it was a sudden drop that coincided with the tire change, and the drop has persisted and stayed consistent. The Goodyears are MUCH "stickier" in hard turns and the car is able to hold it's line much better, unlike the "skating feeling" of the apparently much harder Pirelli compound. I'll take the drop in MPG for the better cornering.
Yes so when I go from 0w-20 to 5w-30 in my Titan there is no change. This particular event is odd and warrants further investigation. I actually added 2psi to my tires when I did the last oil change.


So anyway I'll keep an open mind and look for other factors as I can't believe oil would make such a dramatic change in fuel economy.
 
Yes, thicker oil will result in lower fuel economy and not just by 0.1 mpg. You can easily see a 5% difference between HTHS ~ 3.0 cP vs. HTHS ~ 3.5 cP. The difference becomes more apparent if you drive gently. Your engine will feel smoother and more responsive with thinner oil, too. As one notable member said on BITOG once, go only as thick as necessary and stay as thin as possible or something to that effect.
 
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