Difference in MPG using different oil viscosities?

[CleanSump]

Since I retired my fuel mileage has dropped because most of my driving is now short trips with lots of stop lights instead of 2 hours on uninterrupted highway everyday. When I was commuting it didn't matter if I used a Euro xW-30 or xW-40.

 

[Trashman]

I seem to get between 0.25-0.40 MPG less using a 5w-40 vs a 5w30 but there were some variables that took place so not for certain. Let's say I get 0.25 miles per gallon less, 20 MPG, & $2.50 gallon average. Over 100k miles that would be $3,125 more dollars.

I know this is an older post, but I stumbled upon this and it jumped out at me. In this scenario the difference between getting 20 MPG and 19.75 MPG is an additional $158.23 over 100,000 miles, so it does cost a bit more, but nowhere near $3,125.00.

 

[fantastic]

I know this is an older post, but I stumbled upon this and it jumped out at me. In this scenario the difference between getting 20 MPG and 19.75 MPG is an additional $158.23 over 100,000 miles, so it does cost a bit more, but nowhere near $3,125.00.

@Trashman Thank you for the correction! I must have fallen asleep at the wheel on this math.

Here's my Volvo real world number actually using a calculator & 0.30 MPG less. Shows only $186.21 additional cost over 100k. The second picture below shows the difference between my 87 octane avg vs 93 Octane I've been using lately.

I've only used 3 tanks of 93 so far so not exactly a good avg fuel cost & MPG but here is the 87 vs 93 so far. Over 100k miles it would cost me an extra $4,547.73.

 

[Number_35]

Just to muddy the waters, quite a few years ago I read Louis Lapointe's blog. (Bright Green?) He was the acetone guy. He was deadset against ethanol, and against thin oil.

He claimed that the engine actually worked harder and consumed more fuel when running a thin oil (which he derisively referred to as "cutting fluid"). (Back then, a thin oil was 5W-20 - I can't imagine what he would have thought of 0W-16, 0W-12, and 0W-8 ...)

Anyway, before one long holiday road trip in 2001, I ran Mobil 1 20W-50 instead of the usual 5W-30. The van* turned in its best-ever mileage for one tank - 35 mpg (Imperial, so around 30 mpg US, or 8 l/100 km). We got good mileage overall for the whole trip.

But it's hard to draw any real conclusions from this.

If the weather was moderate, we would have run the AC less.

If the prevailing western winds died down on the way out and picked up to above normal on the way back, that would have helped a lot.

The point being that oil thickness is only one variable, and not overly significant compared to wind, tire pressure, speed, AC use, and probably lots of other stuff.

* 1990 Mazda MPV 2.6 l SOHC inline-4 with 5-speed manual. Around 320,000 km (200,000 miles) at the time. Heavily loaded with camping gear.

 

[kschachn]

I laid out all of the details in that post. Never claimed to have standardized anything?

Exactly.

 

[Trashman]

@Trashman Thank you for the correction! I must have fallen asleep at the wheel on this math.

No problem, I’ve done that where I do a math problem backwards or forget one step or something and the answer come out way off and I know I messed something up That 93 octane sure is a hit to the wallet!!! Am I the only one who remembers when it used to be a 10 cent jump between grades? Now it’s 40, 50, 60, 70 cents between grades at some stations. Pure greed.

 

[Trashman]

Just to muddy the waters, quite a few years ago I read Louis Lapointe's blog. (Bright Green?) He was the acetone guy. He was deadset against ethanol, and against thin oil.

He claimed that the engine actually worked harder and consumed more fuel when running a thin oil (which he derisively referred to as "cutting fluid"). (Back then, a thin oil was 5W-20 - I can't imagine what he would have thought of 0W-16, 0W-12, and 0W-8 ...)

Anyway, before one long holiday road trip in 2001, I ran Mobil 1 20W-50 instead of the usual 5W-30. The van* turned in its best-ever mileage for one tank - 35 mpg (Imperial, so around 30 mpg US, or 8 l/100 km). We got good mileage overall for the whole trip.

My experience has always been exactly like your’s. I log my mpg every fill up, and everything from the RAV4 Hybrid on up to the K1500 gets overall better MPG’s when running thicker oil. I know I’m in the minority here, but my personal experience has been that engines run quieter, smoother and get better MPG’s on thicker oil, and yes indeed, synthetic oil makes an older non-leaking engine develop leaks. I know, I know, I’m a crazy conspiracy theorist
But us crazy conspiracy theorists have been batting 1.000 lately, so I’m sticking with my thicker oil

 

[Number_35]

My experience has always been exactly like your’s. I log my mpg every fill up, and everything from the RAV4 Hybrid on up to the K1500 gets overall better MPG’s when running thicker oil. I know I’m in the minority here, but my personal experience has been that engines run quieter, smoother and get better MPG’s on thicker oil, and yes indeed, synthetic oil makes an older non-leaking engine develop leaks. I know, I know, I’m a crazy conspiracy theorist
But us crazy conspiracy theorists have been batting 1.000 lately, so I’m sticking with my thicker oil

I can't say that the thick oil helped my mileage - perhaps with a 5W-30 we would have racked up 36 mpg instead of 35 ... or perhaps 34.

It wasn't a controlled experiment, and there were way too many variables.

But I can say with reasonable confidence that if the thicker oil hurt, it sure wasn't significant.

 

[fantastic]

Exactly.

Exactly what? You insinuated that I used a "Standard Fuel Test". Why did you ask? Did you expect any different answer than what I already said? You asked but got the same answer twice. It's okay sometimes I need to be told twice on things too for it to sink in.

 

[fantastic]

No problem, I’ve done that where I do a math problem backwards or forget one step or something and the answer come out way off and I know I messed something up That 93 octane sure is a hit to the wallet!!! Am I the only one who remembers when it used to be a 10 cent jump between grades? Now it’s 40, 50, 60, 70 cents between grades at some stations. Pure greed.

It's my Schmottness that got the best of me!!!!
Yes, In the 90's I remember Casey's & Break-Time fuel stations only being a small difference. Matter of fact I remember those stations charged the exact same for 87 & 89 octane.

 

[kschachn]

Exactly what? You insinuated that I used a "Standard Fuel Test". Why did you ask? Did you expect any different answer than what I already said? You asked but got the same answer twice. It's okay sometimes I need to be told twice on things too for it to sink in.

"I seem to get between 0.25-0.40 MPG less using a 5w-40 vs a 5w30 but there were some variables that took place so not for certain."

Just the fact that you even think such a small difference is possibly due to the oil.

 

[tblt44]

None you can measure

 

[ineptizoid]

I’ve run 5w30, 10w30, and 10w40 in 2 different Nissan Frontiers and a Honda Fit over the past several years…never could tell any difference at all in fuel mileage between viscosities, and that was using a calculator, not guesstimating. And this is over the course of almost 300K miles between the 3 vehicles. If oil viscosity made any difference, it was less than a tenth of a MPG. I’m sure tire PSI makes a bigger difference than that.

 

[Oliveoil2]

Tire psi and tire design play in to it as well. I got my best fuel mileage and wear (80k) out of a set of OME Bridgestone Ecopias. Such a great tire. Yet they were almost 40% more than the General Altimax 365 which are 20% less fuel efficient, but a great value. 0/5w-20/30 really doesn’t matter for mpg. It’s how one drives that makes a difference. Always easy on the gas or the brakes, coasting to most stops.

 

[fantastic]

"I seem to get between 0.25-0.40 MPG less using a 5w-40 vs a 5w30 but there were some variables that took place so not for certain."

Just the fact that you even think such a small difference is possibly due to the oil.

How many OEM's have gone to lower oil viscosity? For what reason?

 

[kschachn]

How many OEM's have gone to lower oil viscosity? For what reason?

Which has nothing to do what you postulated earlier. 0.25 -0.40 measurement difference in the real world due to the oil is stupendously indeterminate.

 

[fantastic]

Which has nothing to do what you postulated earlier. 0.25 -0.40 measurement difference in the real world due to the oil is stupendously indeterminate.

I mentioned there were other variables & I wasn't certain how much was due to oil...right? Otherwise, Are you implying my manual is wrong about fuel economy & oil grade for my engine?

 

[Dave Hess]

I personally have never noticed a measurable difference in MPG using different oil viscosties, but I was curious if anyone else has. My examples are small four cylinder economy cars so maybe that's my issue. My son's 2012 Corolla has used 0w20 all the way up to 10w30 high mileage oil and the MPG's seem the same. My current 2024 Corolla came with 0w8 and currently has 0w30 Mobil ESP, I'm averaging about 42 MPG highway on the 30wt oil.

I rented a 2024 Corolla and was doing 37 MPG on highway at 80 MPH through Oregon hills and mountains.

I could also match 42 MPG on flat surface at 65.

Great car.

 

[barryh]

Since I retired my fuel mileage has dropped because most of my driving is now short trips with lots of stop lights instead of 2 hours on uninterrupted highway everyday. When I was commuting it didn't matter if I used a Euro xW-30 or xW-40.

I think that logically the biggest mpg difference vs oil grade is going to be in short tripping where the oil never gets up to temperature. In these circumstances a thinner oil and particularly a lower W rating can have a measurable effect. I commuted 18 years on an air cooled motorcycle and recorded every drop of fuel that went in it. The mpg graph is a recognisable sine wave with dips in the winter and peaks in the summer. I know several other factors contribute such as seasonal fuel blend and air density but all the oils - engine, transmission, final drive will be producing more drag in the winter and will therefore benefit from thinner grades or synthetics with a high VI.

 

[kschachn]

I'm

I mentioned there were other variables & I wasn't certain how much was due to oil...right? Otherwise, Are you implying my manual is wrong about fuel economy & oil grade for my engine?

Yeah sure, that's it.