Highway hypermiling: a false economy?

Does it make sense to drive slower, if time has (monetary) value? #

Ever since driving a car equipped with a fuel economy meter, I have become pretty interested in conserving fuel while driving. (Obviously, the best way to conserve fuel is to not drive at all.) Not surprising, the best way to do this is to minimize the use of the gas pedal.

Turns out there’s a whole community of people called hypermilers doing this for various reasons - reducing emissions, cutting fuel costs, and simple pride at getting a better MPG figure than advertised. Hypermilers advise driving no more than 90 km/h, since fuel efficiency tends to drop pretty quickly above this point due to increased drag, both aerodynamic and mechanical.

Let’s compare the cost savings in fuel to the increased time spent on the road, based on the following assumptions:

• the main reason to conserve fuel is to save money
• time spent driving has an equivalent dollar value

Optimum highway speed for minimal cost #

Since we are trying to save money, we want to minimize both the money spent on fuel, and the money “spent” on time. The optimum speed can be found by minimizing the total cost per km:

TimeCostPerKm = TimeValue / Speed

FuelCostPerKm = CostPerLiter * FuelEfficiency / 100

TotalCostPerKm = TimeCost + FuelCost

Fuel efficiency depends on the speed at which you are travelling, as shown in the graph above. I made a number of comparisons based on those trends, for both low consumption (small diesel car) and high consumption (large gasoline van) vehicles, with fuel cost set to $1 / L. My conclusions follow. Note that these are based solely on highway driving at a constant speed.

Low consumption vehicles: drive as fast as you can #

Even at a very low time value ($10/hr), the optimum speed for the low consumption vehicle is 120 km/h, well above the given recommendation. Increase the time value to a more realistic $40/hr for those who could afford such a vehicle, and the formula says that you should basically drive as fast as you can.

High consumption vehicles: it depends how much your time is worth #

At my low time value figure of $10/hr, the optimum speed for the high consumption vehicle - which takes 4 times the amount of fuel as the car - is the recommended 90 km/h. But at $40/hr, the optimum speed again reaches up to 120 km/hr.

What if fuel costs quadroupled? #

At the $40/hr time value, the higher speed of 120 km/hr would still be optimal for the small diesel car, but the fuel cost begins to dominate for the large van, giving an optimal speed of 90 km/hr.

Hidden costs and caveats #

I assumed that the main reason to conserve fuel is to save money, but burning more fuel also has an environmental cost. How much does it cost, in dollars, to counter the environmental effects of producing the fuel, as well as emissions from burning it? This is a very complicated question, and I would love to have a ballpark number to use in my calculation above.

In addition to fuel cost, driving your car at a higher speed may increase wear and tear. Violating the speed limit may incur speeding tickets, as well as increase the risk and impact of accidents. Finally, the calculation only deals with fuel consumption at a static speed. Maintaining a speed does not take a lot of power, but accelerating to that speed does. You would have to accelerate more to reach the higher speed. Driving at a higher speed also incurs more braking - for instance, when traffic slows - which in turn incurs more (costly) acceleration.

My second assumption involved the value of time. In some situations, time has a specific dollar value. Imagine your employer is paying you to drive across town to another office. The amount of time you are on the highway is paid, yet unproductive. In this case, it is in your employers best interest to minimize the combined time and fuel cost per km, based on your hourly wage. For the self-employed person, travelling between paying gigs, the time is unpaid, yet is directly cutting into their billable hours. It would pay for this person to optimize based on their billable wage.

Then again, is driving really so bad that it gives the driver no value at all? Some people enjoy driving. Either the employee or the self-employed person may view it as a break between periods of productive work. These breaks may have value in work terms, if they increase productivity on average. If you are working a single salaried job, your commute to and from work simply cuts into your earnings - you are not trading billable time for driving time. It is up to each person to determine the value of leisure time, and whether driving counts as leisure time. If the leisure value of driving is $5/hr but you value watching TV at $10/hr, the time cost of driving is then $5/hr. What if you prefer driving to being at home? Maybe the time cost is negative! Maybe time cannot really be valued in dollars at all?

Finally, this calculation is not meant to be a condemnation of hypermiling techniques generally, only a critique of the recommendation that you drive slowly on highways. Many ‘city techniques’, such as timing your approach to lights, will save fuel without costing you any time. Keep optimizing!