One thing I’ve always wanted to know is, what is the ideal speed to drive your car for maximum fuel efficiency?  Obviously aerodynamic drag is going to decrease fuel economy at higher speeds, but I always suspected there was a speed point below which other factors also reduced fuel economy. 

In the past I had tried recording long drives and then plotting instantaneous fuel economy versus vehicle speed, but I always suspected the data was being corrupted by coasting and accelerating.  The lower speeds tended to be either accelerating or coasting while faster speeds tended to be primarily steady state cruising.

To rectify this I found a stretch of road on the way home from work with minimal traffic so I could vary my speed without being a traffic hindrance.  Each day I drove the same stretch of road at different speeds using the cruise control.  I used the Torque app on my phone with a Bluetooth adapter connected to the car’s OBD port to record the fuel economy and vehicle speed.   Since the software can also use the phones GPS I was able to only use data that was recorded between specific latitude points so the same start and end points were always used.

As can be seen in the chart below my particular car appears to have the best steady speed fuel economy at about 35 mph.  I suspect some others factors are probably included in this data, with the largest suspect being ambient air conditions, probably air temperature or possibly humidity.  After I get some more data over a whole year I plan on making separate curves for different temperature ranges.