How much does oil viscosity actually impact MPG?

[Gene K]

Your stuck on door sticker for a tire size and speed rating that no longer exists on either car…

The OEM tires on the Saturn were 185/65R15 with 30 front / 26 rear, with 35 PSI max, “T“ speed rated…
Current 51 PSI max, 205/60R15, “H” speed rating.
Put 26 in the rear on the current tires, this is reckless, and dangerous.

I do inflate hard for mileage and tire wear, 32f / 40r would be my normal pressure

Typical load ratings
185/65R15 T = 88
205/60R15 H = 91

Load ratings are established at 35 (US) or 36 (Euro) psi for standard load tires as are the speed ratings. The maximum allowed cold inflation pressure is for special circumstances and to allow vehicle manufacturers to make handling adjustments.

The new tires should ideally have a lower pressure not higher because of increased load rating at the same pressure. However the minimum inflation pressure has been set at 26 psi for radial tires so the factory pressures are the best he can do while attempting to maintain factory handling bias. Running excessive rear pressure can be dangerous because it effectively increases rear spring rate possibly causing oversteer (rear of car losing traction first) in an emergency maneuver.

If you choose to run more pressure than oem attempt to keep the bias similar. Say 40 psi front and 35 psi rear to keep the handling bias more toward design unless you have experience adjusting handling bias and a safe place to test breakaway characteristics. Be aware that this may reduce the vehicles cornering limits and increase braking distance.

Note: Tire manufactures are currently bringing uniformity to the tire rating system so all standard load tires are going to 36 psi for the max load rating as they are redesigned.

 

[ryanschillinger]

Wait. Did someone say a 10w30 is same as a 0w16 after 10 hours driving?

 

[CharBaby]

I believe that a thinner oil will do its own small part just like all of the other fuel saving benefits such as:

*Lower vehicle weight
*Lower Cd
*LRR Tires
*Easy/easier driving
*Proper tire inflation
*Stop/Start
*Combining trips
*Well engineered engine/tranny combo/gear ratios
and many more...

Each of these benefits all by themselves/on their own, may not show any significant benefits to the owner.
However in combination with all of them together shows significant fuel saving benefits. If 1, 2 or 3 MPG is considered significant.

I mean, if you ONLY add or removed the roof rack from a vehicle, you may not see any difference. I tried it on an SUV and never saw a single bit of difference. So, I put it back on 'cause I liked the way it looked and I never, ever put anything up there.

I have tried 0W20, 5W20 & 5W30 and all by themselves, I didn't see any fuel economy differences winter or summer. I mean, I'm sure there's something, I just didn't see it.

I've changed tires and never really saw any difference with just tires alone. I may have heard/felt differences but, I didn't see any fuel economy differences +/- that I could measure.

The only think that makes a noticeable difference in my own personal testing is octane. The higher the octane, the worst my fuel economy was or no difference. Nor in power output that I could feel or measure with a stopwatch/timer. But I'm sure others would disagree with their own vehicles.

 

[ka9mnx]

Running these experiments in three of my vehicles. No measurable difference between 0w/5w-20 and 5w-30 synthetic. Now we are trying 10w-40. I think if we had colder Winter's I might see something. I run these oils for more than one year to account for temperature and fuel formulation differences.

 

[ZZman]

I agree with most here that tire pressure and driving style is more important for mpg than oil viscosity. .5 mpg at best.

 

[kschachn]

Running these experiments in three of my vehicles. No measurable difference between 0w/5w-20 and 5w-30 synthetic. Now we are trying 10w-40. I think if we had colder Winter's I might see something. I run these oils for more than one year to account for temperature and fuel formulation differences.

This post is about fuel but it is applicable to oils as well. The fuel economy difference between a 20 and a 30-grade is deep in the noise of everyday driving. No one on here could ever attribute an observed difference to one isolated variable. Attribution is always the problem, not necessarily measurement. You may think you're controlling variables but there are many, many more that you are not.

Best Brand of Gasoline

Get all my Premium at Sam's Club. Other places around here are 40-50 cents more expensive than Sam's Club premium, making it well worth driving a couple extra miles to fill it up. Get my 87 wherever it's convenient.

bobistheoilguy.com

 

[IndyIan]

I was just thinking that maybe with extended cold start warm ups and short trips, like idling the car for 5 minutes then driving for 10 minutes, you could probably see a significant difference in the amount of fuel used with thicker or thinner oil. If you do whole tanks of this type of driving. Obviously idling the car for a minute less each time would make much more difference, but some wives don't like that type of logic...

I could have put the scangauge on the Outback for a comparison fuel measurement for a cold start at -5C and 10 minutes of idle. 6k mile old 0W20 vs new Mobil synthetic 5W30. My impression is that the engine quieter at idle and low rpms with the 5W30, but that's what I want to hear, so who knows.

 

[ka9mnx]

This post is about fuel but it is applicable to oils as well. The fuel economy difference between a 20 and a 30-grade is deep in the noise of everyday driving. No one on here could ever attribute an observed difference to one isolated variable. Attribution is always the problem, not necessarily measurement. You may think you're controlling variables but there are many, many more that you are not.

Best Brand of Gasoline

Get all my Premium at Sam's Club. Other places around here are 40-50 cents more expensive than Sam's Club premium, making it well worth driving a couple extra miles to fill it up. Get my 87 wherever it's convenient.

bobistheoilguy.com

OH, sorry. I thought we were discussing oil viscosity and MPG. And thanks for explaining what I already said.

 

[CarbonSteel]

Proper maintenance, tire pressure, and right foot control will impact fuel economy more than oil viscosity ever will.

A head wind will impact your MPG more than a change from 15W-50 to 5W-30...

 

[wag123]

Why not just split the difference and go with 0W-30 for the win/win?

 

[kschachn]

Why not just split the difference and go with 0W-30 for the win/win?

Except that the winter rating is not directly connected to the operational viscosity.

 

[OVERKILL]

Your stuck on door sticker for a tire size and speed rating that no longer exists on either car…

Stay in the boat. The OP didn't indicate he was running different tires than stock, you gave your advice, then added a 2nd post detailing that you and dear 'ol dad are doing that, hardly relevant to the "advice" you handed to the OP.

The OEM tires on the Saturn were 185/65R15 with 30 front / 26 rear, with 35 PSI max, “T“ speed rated…
Current 51 PSI max, 205/60R15, “H” speed rating.
Put 26 in the rear on the current tires, this is reckless, and dangerous.

I do inflate hard for mileage and tire wear, 32f / 40r would be my normal pressure

We've been over what would be appropriate tire pressures for your Saturn in a previous thread, it has absolutely no relevance to this one, nor the blanket, and awful, advice you attempted to provide here, end of story.

 

[Tom NJ]

The change in MPG from increasing or decreasing an oil viscosity by one grade is very small, usually less than 1%. You cannot measure this in your personal vehicle. You may think you can, but you cannot isolate the effect of oil from the many other variables occurring with each fuel tankful. The only way to determine the effect of the oil grade is through carefully designed tests that control and standardize the multitude of other variables, such as the Sequence VI test.

Here is a graph of MPG from my past 2008 Altima over about an eight year period. I only used Mobil 1 5W-30 in this engine, and the vast majority of the gas tank fill-ups were with the same grade of gas from the same Amoco gas station. Each point on the graph is a calculated MPG from the actual mileage and gallons from each fill, and range from 18 to 33 MPG, that's +- 7.5 MPG or +- 30%! Think you can pick out a 1% change (0.25 MPG) from an oil grade change? Think again.

A 1% improvement in MPG equates to less than 1/2 a gallon of gas a month for the average driver. Who cares! No sense trying to measure it, just accept that it is there and move on to controlling the other more significant variables noted by others above.

 

[Astro14]

Went from 0W20 TGMO to 5W30 PUP in my Tundra.

Highway MPG improved slightly.

I track every tank. I make the same run (up the Eastern Shore to Newark) frequently.

I made the switch when the truck had 5,000 miles on it.

I suspect that the improvement was more a function of engine break in than of any oil effect.

Of this I’m reasonably certain: oil viscosity makes so small a difference in MPG that it is outweighed by every other factor: tire rolling resistance, tread design, and inflation, driving habits, traffic, wind, slope, and load.

 

[kschachn]

Never mind.

 

[Accent Abuser]

Depends on how you drive it more than anything. If you do lots of short trips in the winter you'd probably see a 2-3mpg impact between a 5w40 and a 0w20. The difference between a 0w40 and a 0w20 or 0w40 and 5w30 would be even less.

Anecdotally, I find the winter viscosity makes a bigger difference than the operating viscosity. I get better fuel economy using a 10w-40 than a 15w-40 with similar HTHS and KV100.

If you want the best trade off between protection and fuel economy, I'd go with a 0w40 or 0w-30.

 

[kschachn]

Depends on how you drive it more than anything. If you do lots of short trips in the winter you'd probably see a 2-3mpg impact between a 5w40 and a 0w20. The difference between a 0w40 and a 0w20 or 0w40 and 5w30 would be even less.

Anecdotally, I find the winter viscosity makes a bigger difference than the operating viscosity. I get better fuel economy using a 10w-40 than a 15w-40 with similar HTHS and KV100.

If you want the best trade off between protection and fuel economy, I'd go with a 0w40.

And how are you determining that? Please don't say by your ad-hoc everyday driving because that would make it an even bigger bunch of nonsense than claiming to attribute some change to a grade. Read Tom's post above and see if you still think this is true.

And it's not a "winter viscosity".

 

[Accent Abuser]

And how are you determining that? Please don't say by your ad-hoc everyday driving because that would make it an even bigger bunch of nonsense than claiming to attribute some change to a grade. Read Tom's post above and see if you still think this is true.

And it's not a "winter viscosity".

I said anecdotally, because I have no way to scientifically determine mpg changes. W stands for winter, so yes, that the winter viscosity, or more accurately measured cold cranking and pumping temperature limits.

Also as I stated, the change is incredibly small. I always advocate on here that if engine protection and longevity is important to you, then you should always go with a higher viscosity. Gas might be $5 a gallon, but that sure as hell isn't going to stop me from using 15w-50 this spring and summer. Engines are expensive, and no gas savings from using 5w-20 or 0w-20 are going to save me enough money to replace an engine that's worn out from inadequate viscosity.

 

[weasley]

As @Tom NJ pointed out, the noise in the data is far bigger than the small benefit a change of oil might make. Weather, traffic, topography, tyre inflation, tyre wear, wheel alignment, vehicle load, and - the biggest influence of all - driver behaviour all play bigger parts than oil viscosity and there's no way you will level all of these variables and pick out the oil's influence.

 

[JohnnyJohnson]

As @Tom NJ pointed out, the noise in the data is far bigger than the small benefit a change of oil might make. Weather, traffic, topography, tyre inflation, tyre wear, wheel alignment, vehicle load, and - the biggest influence of all - driver behaviour all play bigger parts than oil viscosity and there's no way you will level all of these variables and pick out the oil's influence.

BINGO!