Efficiency

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Improving the efficiency of traveling on a Zero has a significant effect on range. The powertrain efficiency is high enough (few internal losses) that most gains should focus on aerodynamic drag reduction and rolling resistance.

Aerodynamics

Basic Recommendation
Motorcycle aerodynamics is a complicated topic, and most riders should focus on a simple windscreen that improves comfort and keep their body behind that.
Going Further
To go further, the entire shaping of air around the vehicle and rider has to be considered, especially the transition from laminar flow to turbulent flow around the front of the vehicle, then the rider, and around the tail.
Drag
Drag is related to the vehicle's airspeed, not just its groundspeed.
Any headwinds, sidewinds, or (lucky) tailwinds will impact the design significantly.
A robust design can handle a lot of variation and not lose performance.
Illustrations
Theory References
Drag coefficient
Drag equation
Motorcycle References

Crouch

Crouch down! Behind a windscreen, if possible, or avoid a crouch by using a windscreen.

Windscreens

Windscreen Adjustment
A good third-party windscreen when adjusted properly should provide a 5-10% increase in range.
Rider goals are some combination of comfort (from wind and weather) and better highway range.
Increased noise from wind with a windscreen is unfortunately easy to achieve, and often primarily due to airflow striking the underside of the helmet near the ears.
Generally, attempt to move the windscreen as forward as possible to make the most of the small/short Zero cockpit area.
Deflect the air as vertically as possible for the rider's height, but mind that air tends to get turbulent if too much energy is put into it in too small a space, resulting in turbulence pushing the rider around at high speeds. A good position helps achieve a balance.
A laminar lip or spoiler on the top edge of the windscreen will soften up the airflow to minimize buffeting and sculpt the air around the top of the helmet.
References

Total Cross Section

The coefficient of drag is a function of the shape of the motorcycle and rider combined.

  • A shorter motorcycle presents less front cross section than a taller motorcycle.
  • A shorter windscreen is, other things being equal, better than a taller windscreen.
  • Shorter handlebars:
    Replacing the straight-through bars with clip-on bars or the like would reduce some of the frontal area.
    Remounting the Zero control assemblies on new bars takes a bit of drilling and special Torx drivers (T-22) so plan before you try this.

Front

Oncoming air should be smoothly curved around most of the rider's body.

Aft

Air as it passes any fairings or the rider's body will detach if the transition is abrupt for the current velocity.

Wheel Covers

Covering the wheel spokes reduces drag by about 5%, because the wheel is moving forward into the airstream, and the upper half of the wheel can be moving as much as twice the current vehicle airspeed.

Material
Wheel covers should be made of a relatively soft plastic or a smooth aluminum "moon" disc.
ABS plastic works for the rear wheel.
The front wheel requires a very soft plastic (like Coroplast or such) to take the impact from rocks and gravel kicked up from other vehicles so that the cover doesn't crack or shatter.
Shape
Wheel covers should sit concave within the rim to avoid marring the tire or interfering with the brake calipers.
A cutout on one side is worthwhile where the tire air valve sits to keep it easy to access.
Fastening
For DIY plastic jobs, multiple zip-ties made from an outdoor, UV-resistant plastic can do a reasonable job.
For a sturdier mount, bolts of sufficient length with nylon locknuts and broad nylon washers will hold covers in place more permanently and would be easier to remove and replace repeatedly.

Rolling Resistance

Rolling resistance relates to the wheel bearings (sliding friction) and the tire contact patch with the road (rolling friction).

Tire Tread

Broadly, a more street/touring oriented tire will offer less rolling resistance than a knobbier off-road tire.

Compared to the stock DS tires, 90/10 or sport-touring tires might net about 5% extra range.

Tire Pressure

Higher tire pressure will reduce rolling resistance for any tire, thus providing a minor (5%) increase to range, but does raise the tires' sensitivity to rough roads.

Carrying a portable (12V or 120V) tire pump is a solution for wanting to adapt the tire pressure based on what a long trip demands in different sections.