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Synthetics: When they do (and don't) help mileage

7K views 10 replies 6 participants last post by  bstrom 
#1 ·
These charts are from a Cummins white paper on fuel economy (geared toward commercial vehicles, but a good read for anyone interested in maximizing mileage): http://cumminsengines.com/uploads/docs/Secrets of Better Fuel Economy_whitepaper.pdf

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The gist is that synthetics do make a significant difference in axles, differentials, & transmissions (where temperatures are often below ideal operating conditions), but they probably will not "pay for themselves" (mileage-wise) when used in the engine of a vehicle operated with relatively few cold starts...
 
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#2 ·
These charts are from a Cummins white paper on fuel economy (geared toward commercial vehicles, but a good read for anyone interested in maximizing mileage): http://cumminsengines.com/uploads/docs/Secrets of Better Fuel Economy_whitepaper.pdf

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The gist is that synthetics do make a significant difference in axles, differentials, & transmissions (where temperatures are often below ideal operating conditions), but they probably will not "pay for themselves" (mileage-wise) when used in the engine of a vehicle operated with relatively few cold starts...
Good find and thanks for posting. I need to sit down and read through this but if your summary is accurate this confirms what I have suspected but have been unable to accurately test for. I still run synthetics wherever possible in my motor simply from a longevity perspective - I plan to keep my truck until there are at least 600,000 miles on the odometer.

I am curious as to why they tested 5w-30 synthetic against 15w-40 mineral based oil. I think I would have preferred to see 5w-30 tested against 5w-30. Gotta sit down and read the study.

Thanks again for posting.
 
#3 ·
Yeah I wondered why they used a 5w-30, but have to assume a synthetic diesel 5W-30 offers the most gains over a mineral base 15W-40. I do know from working in the trucking industry that fleets still use mineral base oil in engines despite investing thousands of dollars (per truck) for other marginal mpg improvements.

Despite the lack of mpg improvement, I still use synthetic 0W-30 in my own tundra (among the many benefits, it starts so much better at -20).

But I still find the charts useful for putting in perspective the mpg claims often thrown around forums. In a best case scenario (axle/trans temps below 100F) a switch from mineral base oil to synthetics would raise mpg from 15.0 to 15.5 (~3.5%), yet you'll sometimes see on forums claims that switching from one synthetic to another will result in gains of 1.5 mpg (that would only approach the realm of credibility in a vehicle getting 45 mpg).
 
#4 ·
I use synthetics all the way around. As was mentioned, my reason for using synthetic motor oil (Mobil 1 0W30) is for better start up protection and engine longevity. I think on the bottle it shows the savings over a ten year period to be $400, so that pretty much shows the MPG gains aren't much, however if you factor in the longer oil change intervals that are possible using it, it would probably add up to more.

John
 
#5 ·
A month ago I changed oil in my Lexus 2008 to synthetic Castor Edge Titanium 5W-30. It was switched from Chevron 5W-30. Immediately I noticed how MPG dropped - 18-20%. Switching to synthetic oil, I did not wait for a much improvement regarding MPG. I usually was driving about 400 miles per a fuel gas tank, 28 mpg on highway and about 24 mpg in a city. Now with a synthetic oil, my car driving 320 - 330 miles for a fuel gas tank, a make only average 21 - 22 mpg.
My question: is this usual thing with using synthetic oil?
 
#6 ·
No, it isn't, but keep in mind that the gasoline you're using is a winter blend, which will automatically decrease your MPGs. Also, colder temperatures will cut into the fuel efficiency. Not sure what the temperatures have been where you are, but if they've been cold or colder than usual that will cause a drop in MPG's.

A lot of people's driving habits change during the colder weather, ie letting the car warm up, leaving it running while going into a store for a coffee, using drive thru's etc. If you're doing that you're basically getting 0 MPG's while the car is idling.

Finally, you should check the conditions of the spark plugs, air filter, tire pressure etc. Maybe you need a tuneup.

John
 
#7 ·
A month ago I changed oil in my Lexus 2008 to synthetic Castor Edge Titanium 5W-30. It was switched from Chevron 5W-30. Immediately I noticed how MPG dropped - 18-20%. Switching to synthetic oil, I did not wait for a much improvement regarding MPG. I usually was driving about 400 miles per a fuel gas tank, 28 mpg on highway and about 24 mpg in a city. Now with a synthetic oil, my car driving 320 - 330 miles for a fuel gas tank, a make only average 21 - 22 mpg.
My question: is this usual thing with using synthetic oil?
There is no oil that would make that much of a difference in mileage. Something else is wrong or not being accounted for.
 
#9 ·
No, it isn't, but keep in mind that the gasoline you're using is a winter blend, which will automatically decrease your MPGs. Also, colder temperatures will cut into the fuel efficiency. Not sure what the temperatures have been where you are, but if they've been cold or colder than usual that will cause a drop in MPG's.

A lot of people's driving habits change during the colder weather, ie letting the car warm up, leaving it running while going into a store for a coffee, using drive thru's etc. If you're doing that you're basically getting 0 MPG's while the car is idling.

Finally, you should check the conditions of the spark plugs, air filter, tire pressure etc. Maybe you need a tuneup.

John

Thank you for your reply and advise. I also do not want to believe that a good synthetic oil would do such thing and MPG will drop up to 20%. There must be something else. But the MPG drop happened right away after oil was changed. So, something wrong is with oil, I have no other explanations. Today the oil was changed to new Chevron, and after 90 miles driving (mix highway and roads) I see MPG is improving!

I remember the milage I was getting driving last year in winter time - I have not noticed any drop associated with colder weather.

Thanks again.
 
#11 ·
It is very hard to imagine an oil would alter fuel mileage in 90 miles. Synthetics are superior lubricants that retard the progress of oil oxidation and clean motor parts far better than organic oils. Some vehicles have clearances that reduce these advantages.

I've seen dramatic differences in fuel mileage from different stations, especially during winter. Wouldn't be surprised if we are being cheated on octane, etc. over these months. Still, I see no more than a 3-4 mpg variation which is plenty but not catastrophic.

Any number of engine components can affect fuel mileage - plug wires, plugs, PCV valves, air filter, fuel filter, battery connections, etc. all play a collective part in engine performance. Remove these variables with maintenance and mileage will be consistent, but not perfect.
 
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