What the original comparison addressed
Jones and Doust studied treadmill gradients to approximate the energetic cost of outdoor running, including the absence of outdoor air resistance. Their finding addressed that comparison, not a rule that all walkers and runners must use the same incline.
Airflow, treadmill calibration, speed, running surface, and individual movement can all affect the experience. A fan changes thermal comfort without making the belt into an outdoor route.
Decide what needs to match
If your purpose is an accessible indoor aerobic session, the relevant question may be whether the effort suits you. If you are preparing for a particular outdoor event, terrain, turning, pacing, and environmental conditions also matter.
An incline adjustment addresses only part of that difference.
Keep separate records when necessary
“Treadmill, this machine, this incline” gives a more useful comparison than treating its displayed pace as perfectly interchangeable with GPS pace. Outdoor GPS also has measurement error, especially near buildings or under tree cover.
Neither setting invalidates the other. A slower outdoor run or a more demanding indoor run can be explained by conditions without concluding that fitness changed between them. Use the setting that fits the session, and describe the conditions honestly when comparing results.
