Automotive expert Doug Bethune has repeatedly said on CBC radio that one of the most important maintenance items to apply to your vehicle to obtain least fuel consumption is to change the thermostat each year.  I have never changed a thermostat – ever – in my many years of driving.

After reaching operating temperature, at speed varying between 80 and 90 KPH and on level road my Subaru Forester consistently gets 6.6L per 100 KM.(Approximately 42.25 MPG).

Under the same conditions except with a cold engine the fuel consumption is twice above.  This suggests to me that Bethune’s comments that modern engines need to operate at a certain temperature to obtain the best fuel consumption is correct.

What I don’t know is how often a thermostat really should be changed.  How long does it take for a thermostat to lose its ability to maintain a narrow corridor of temperature?