GULLY FUEL USE CALCULATIONS
I was very interested to read the article in the May 26, 2022, issue of the Ka¯ piti Observer by Ben Strang regarding the increased fuel usage by using the Transmission Gully road.
However, by using the laws of physics I would like to suggest that the additional fuel usage is not because of the steeper climbs but because of the higher speeds that the road can be travelled.
To explain this it is necessary to understand that for a vehicle travelling along a road, the energy that the vehicle’s engine produces is required to overcome the forces required to push the vehicle up the slope (if any), the force needed to overcome the mechanical friction in the drivetrain of the vehicle and the rolling resistance of the tyres, and lastly the force to overcome air resistance or aerodynamic drag on the vehicle.
The energy to drive the vehicle up the slope will be recovered by the vehicle when it returns back down the slope to the starting point which would be as the test described.
The comment in the article ‘‘by contrast, it was using almost no fuel when gliding down the steep sections’’ supports this statement. For all practical purposes the force to overcome mechanical friction and rolling resistance is almost constant at any speed.
However, in contrast, the force to overcome air resistance is proportional to the square of the speed of the vehicle, i.e if the speed is doubled from say 50kph to 100kph then the air resistance will be two x two = four times. In the test runs the average speed of the vehicle was 73.2kph over the State 59 route and was 95.7kph over the SH1 route.
Hence the ratio of the higher speed to the lower speed was 1.31 and squaring this it is 1.71 and hence the energy to overcome the air resistance at the higher speed was 71% greater than at the lower speed. Unfortunately it is not quite that simple because, although the SH1 route can be driven at approximately constant speed up to 100kph, the SH59 route traverses sections of road with speed limits of 50, 80 and 100kph.
Each of these would need to be calculated individually and aggregated to provide a more accurate comparison.
However, I believe that the above rough comparison demonstrates that the extra fuel consumption is due to the higher speed and not to the greater slopes.
Perhaps the reporter could rerun the SH1 route at, say 70kph, and see if the fuel consumption is similar to that used on the SH59 route.