How to Reduce Drag on My Baja ATV?

Are you going fast enough, often enough, for long enough that improving aerodynamics will allow a noticeable improvement in either economy or average speed?

The trade-offs that occur when making changes to improve aerodynamics may hurt performance more than the improvement in aerodynamics helps.

Improving the flow around your ATV (for example using pizza pans to make solid wheel hubcaps) will likely result in more total weight and therefor less rapid acceleration. Reducing the frontal area (for example using more narrow, smaller, smooth tires, or modifying the seat to shift your position as a rider to a less upright, more prone position) will likely have significant possibly dangerous effects on performance.

Since you as the rider are directly in the airflow, perhaps a good place to start might be your contribution. Perhaps look to Olympic cyclists for ideas on reducing drag, possibly create a break-away (reduce risk of possible neck injury) attachment that creates a teardrop shape off the back of your helmet.