I investigated two questions:​ 1. Is the preferred walk-run transition speed (PTS) on inclines triggered by minimizing pulmonary ventilation and/or minimizing carbohydrate utilization? and 2. How do walking economy and running economy correlate at different inclines? Ten healthy, high-caliber, male trail/mountain running athletes both walked and ran on a treadmill at various speeds near the PTS at inclines of 0, 5, 10 and 15 degrees. I measured the subjects’ individual PTS and analyzed their expired air to determine their ventilation rate (V̇ _E), overall metabolic rate and carbohydrate specific metabolic rate. The intersection of the walking and running linear regression equations (vs. speed) defined the ventilatory optimal transition speed (VEOTS) and the carbohydrate sparing optimal transition speed (CHOTS). PTS, VEOTS and CHOTS all were slower on steeper inclines. PTS was generally slower than VEOTS and CHOTS at 0, 5 and 10 deg, but converged at 15 deg. Across inclines, VEOTS and CHOTS did not trigger PTS. Within each gait, WE and RE correlated strongly when the difference between inclines was 5 deg (e.g., 0 deg vs. 5 deg), but the correlations weakened at 10 and 15 deg differences (e.g., 5 deg vs. 15 deg). Comparing WE and RE at matched inclines generally yielded moderate correlations (e.g., WE 10 deg vs. RE 10 deg). These results indicate that athletes who expect to both walk and run during trail/mountain races should train with both gaits and on a variety of relevant inclines.