Wheelchairs in various embodiments include a variable-position drive system that can automatically move the drive wheels fore and aft through an approximately 41 cm range. Such a system provides all the benefits of front-, mid-, and rear-wheel drive configurations in a single wheelchair, for example, to adjust the drive wheels toward the rear for stability and speed outdoors and toward the center for maneuverability indoors. The performance-optimal placement of drive wheels often depends on the physical environment (e.g., terrain), on the combined center of mass of the wheelchair and user, and on the user's cognitive functioning, trunk stability, and driving skills. One disclosed embodiment includes a pair of mid-position drive wheels that can automatically be repositioned forward or backward to accommodate changes in terrain and center of mass, which will improve the power wheelchair's stability and maneuverability and, consequently, safety and ease of use in a wider range of scenarios.