The global climate crisis has entered a critical phase, with scientific consensus indicating meaningful emission reductions must occur within this decade to limit global warming to 1.5°C above pre-industrial levels. Road transport remains a major contributor, accounting for roughly 15% of global anthropogenic CO₂ emissions. In Japan, passenger car transports alone generated approximately 101 million metric tonnes of CO₂ in 2019. Okinawa bears a share of this burden with 858,004 registered passenger cars among roughly 1.38 million residents (Stats-Japan, 2024). Applying the US EPA’s standard figure of 4.6 metric tonnes of CO₂ per passenger vehicle per year, Okinawa’s private car fleet alone is estimated to generate approximately 3.95 million metric tonnes of CO₂ annually.This paper presents ”Eco-Mode”, a navigation framework built on OpenRouteService (ORS) to address this challenge. Building on the established relationship between road gradient and fuel consumption (Huang & Peng, 2018), we score per-segment emissions using a tiered gradient-based factor derived from Jiang et al., 2025, applying a routing algorithm constrained by travel time to identify the lowest-emission path. Across 20 routes spanning short urban trips to inter-city journeys, the Eco Drive route achieves a mean CO₂ reduction of 9.8% (median 10.75%), with a mean travel time increase of 5.3%. Given a fleet of one million vehicles statewide, even partial adoption could yield reductions of thousands of tonnes of CO₂ annually at negligible infrastructure cost.