This work presents the complete design and construction of a high-efficiency electric powertrain developed for a Shell Eco-Marathon Urban Concept vehicle. The system integrates a 1500 W, 48 V brushless DC motor, a modular two-stage chain drive, and a custom 12S8P lithium-ion battery pack. Mechanical components are supported by CNC-machined mounts and adjustable tensioning systems, while electrical subsystems include a DC-DC converter and provisions for a custom motor controller. To validate the system's performance, a Simulink-based vehicle dynamics model was developed, incorporating a PID-controlled drive cycle, battery state-of-charge tracking, and drivetrain load modelling. Results indicate energy consumption of only 20.95 Wh per lap under competition conditions, demonstrating high efficiency and compliance with technical regulations. The hardware is documented with detailed build instructions, CAD models, and a full bill of materials to support reproducibility. This contribution provides a validated, low-cost, and replicable electric powertrain for ultra-efficient vehicle platforms. The system adhered to Shell Eco-marathon voltage and packaging regulations, balancing weight, manufacturability, and cost. The findings validate the feasibility of a lightweight, cost-effective powertrain for competition and real-world low-speed electric vehicles. The paper concludes with design insights, simulation validation, and recommendations for future optimisation and control strategy development.