The ONLY Degreed Mechanical and Truck/Automotive Engineers on the net with 24 years experience specializing in engine, powertrain/chassis engineering, performance and synthetic motor oil lubrication and filtration.

Gauging Fuel Economy Improvements With Synthetic Oil 

One of the most common questions I get asked is how much fuel economy improvement people can expect to see when they make the change to synthetic oil. I am reluctant to provide any exact numbers because there is no exact percentage increase that will be realized under any given set of operating conditions with different vehicles and vehicle conditions. I can provide a general range of 2 to 8%, for example, or I can state some specific mileage increases that other people have realized or state the results of actual field and fleet tests. However that does not in any way mean that you will see the same values. 

There is documented lab and fleet test data that shows improvements of 4-5% on average better fuel economy with a synthetic 5W-20 vs. 10W-40 and 15W-50 petroleum oils (u) as well as improvements of 8.2% more miles per gallon when synthetics were used in both the engine and drivetrains in a diesel truck test (ii), for example.   In order to see exactly what was included in this testing one must review every specific detail involved in the testing. Unless every parameter is replicated it would be next to impossible for you to realize the exact same values that I provided to you, and if you didn’t notice those values then you would believe I gave you inaccurate information, however that is not the case when you look all the different parameters that affect fuel economy of any vehicle at any given time. 

In my engineering position at a major automotive manufacturer we regularly run fuel economy testing in order to establish the fuel economy numbers that you see on the window stickers new vehicles.  These numbers take many hundreds or even thousands of hours of testing using hundreds of vehicles and millions of dollars of equipment and facilities in order to provide an accurate representation of what the customer can expect to see. The U.S. Federal Government also randomly purchases new vehicles all across the country and subjects them to a similar battery of fuel economy certification tests to ensure the numbers are accurately represented by the vehicle manufacturer. Here is an overview of some of the more common parameters are measured, monitored and controlled during testing at the vehicle manufacturer in order to establish fuel economy ratings: 

-         fuel quality and specific type of fuel, tested and recorded

-         barometric pressure

-         ambient temperature

-         ambient wind velocity and direction

-         wheel torque

-         exhaust gas temperature

-         engine compartment temperature

-         manifold vacuum

-         inlet air temp and velocity

-         air velocity and differential vacuum upstream and downstream of the air filter

-         cylinder head temperature

-         water (coolant) temperature

-         tire type, compound and pressure

-         tire evolutions per mile (varies considerably with different brand and type tires of the same dimensions)

-         suspension settings, spring rates and options

-         fuel flow rate

-         fuel pressure

-         wind velocity while driving using special instrumentation mounted at several different locations on the vehicle body and underbody

-         type and viscosity of lubricants used in the engine, transmission, transaxle and differentials

-         test weight of the vehicle, both single passenger, simulated second passenger and/or maximum GVW

-         brake caliper drag

-         brake rotor lateral runout ( LRO)

-         brake rotor differential thickness variation (DTV; can generate LRO and brake caliper drag over a period of time)

-         brake rotor and lining temperatures

-         brake line pressure in each corner of the vehicle

-         brake pedal force used to decelerate and stop the vehicle

-         parking brake cable loads (possible drag on system) and cage clearances of parking brake linings for drum in hat and drum brake type systems

-         axle lube type, viscosity and temperatures

-         transmission fluid type and temperatures

-         rate of acceleration and loads on pedal

-         emissions system performance

-         chassis dynamometer testing

-         parasitic losses from engine accessories

-         external exhaust system and muffler temperatures

-         plus many, many more parameters 

I could easily continue on and fill up additional pages with the various parameters that all have an effect on fuel economy, however that is not my intent. My intent is only to show you how many different critical parameters all have an effect on overall fuel economy and that any single variable or multiple variable changing can have a significant effect on the overall fuel economy. 

Now, lets say that you determine that your vehicle is getting a specific MPG using petroleum oil, and that you believe the value is fairly accurate because you have monitored it over a period of time. Then you change to a synthetic engine oil and on the next tank of fuel determine your fuel mileage has not changed, went down or went up a specific amount. Then you call me and tell me that you changed to a synthetic oil and  want to know why, for example, you see your fuel economy has not changed, decreased or not increased as much as you expected. You immediately assume that it was the synthetic oil that caused the change or lack of change. However, can you accurately say that all the other parameters, such as in my list above, have remained exactly the same? The answer is no you cannot. You have no way of knowing unless you also have the parameters of the critical items in my list identified and measured both before and after. 

Unless you have your vehicle completely instrumented to measure these parameters you have no way of knowing, and there are very few, if any, average motorists that have access to such equipment and the knowledge how to use it. The closest approximation you can do to determine an average value for the specific change you have made is to continue driving your vehicle as you normally do over a significant period of time covering many different ambient temperature and driving conditions and fuelings. 

Even if you say you purchase the same type of fuel from the same station, the facts are that you do not always get the exact same fuel quality or type. Fuel refiners and manufacturers change the blending and percentages of certain key components depending on the time of year, location of the country the fuel will be used at and the specific base the fuel is manufactured from. You have no way of knowing. They fuel may meet a certain minimum RON or MON (Research Octane Number or Motor Octane Number) however there can be different percentages of additives which can easily cause fuel economy to change. This is only one example. There are many more. 

My point is that when changing to a synthetic oil you should certainly see some fuel economy increase, and you may see it right away, or you might not. The fact of the matter is that you need to give it thousands of miles over many different driving conditions and seasons and/or weather conditions to accurately gage the effect the synthetic had on overall fuel economy, and your vehicle and engine need to be in mechanically good condition. Research and testing clearly indicates that synthetic oil provides improved fuel economy. How much fuel economy varies significantly based on the parameters I have identified. 

Additionally, many vehicles PCM’s (Powertrain Control Modules) are also programmed based on actual measured data to sample specific values from the many different engine sensors and make changes based on the measured parameters from the various engine and powertrain sensors. Some examples are listed below and are not exactly the same on all vehicles. (Note that I do not intend to go into a detailed description of how each sensor interacts with the PCM and engine operation and how it affects fuel economy): 

-         AC pressure switch

-         Brake lamp switch (informs the vehicle when to disengage the torque converter)

-         Camshaft position sensor (tells engine when to fire)

-         Clutch pedal position switch

-         Intake air temperature sensor (provides info to PCM which uses data to operate the EBP, Exhaust Backpressure Valve, which changes engine RPM while idling

-         Transmission control switch

-         Vehicle speed sensor  (uses the signal to control fuel injection, ignition control,  transmission shifting and torque converter clutch scheduling)

-         Accelerator pedal sensor, (calculates fuel quantity) for vehicles w/o accelerator cables

-         Idle validation switch

-         Exhaust backpressure sensor

-         Injection control pressure sensor

-         Engine oil temperature sensor

-         Analog manifold absolute pressure sensor (determines engine load to calculate fuel quantity required)

-         Barometric pressure sensor (calculates ignition timing)

-         Injector driver module- fuel delivery and timing

-         Injector driver module feedback (confirms proper timing/duration of the PCM command)

-         Exhaust back pressure regulator

-         Injection pressure regulator

-         Tachometer output – provides signal to the PCM and is representation of the camshaft position sensor

-         Manifold air temperature 

These engine sensors have a direct effect on fuel economy and there are also many other variables that go into the PCM calculations. These changes take a period of time to take effect and see the overall result. For example, when changing to a higher flow air filter or a different oil certain sensors have to measure the input values, send the signal to the PCM. The PCM has to determine that these values are different than previous signals and make changes to output signals that control engine operation and fuel delivery. This is an extremely simplified description of the process. In reality it is very complex and beyond the scope of this website. 

Scientific data acquired over many years by automotive manufacturers, oil manufacturers and other research and testing facilities clearly show that synthetic oil allows for an increase in fuel economy. In a simplistic description, less resistance to internal drag caused by synthetic oil due to its uniform molecular structure (under extremes of operating temperatures) allows for less fuel input to achieve the same power output. Think of it this way: say you were pulling a heavy load from point A to point B. If you are pulling the load on a rough concrete surface, your body is going to burn more calories to get the load from point A to point B than if you were pulling the load on a surface with a lower coefficient of friction, such as snow or ice. Same exact theory holds true for synthetic oil. It’s a proven fact based on physics and engineering tests and data. 

Therefore, in summary, when you change to a synthetic oil, or make any other engine, component or vehicle changes you need to allow for the changes to be accurately gauged over a period of time while being aware of all other parameters that have been proven to have an effect on fuel economy.