I had a Vivosmart 5 which has been returned for a gps error. It tracked walks etc fine but gave a stupidly high mileage. In the interim I’ve got a vivosmart 4. Last week I did a walk with my vivosmart 5 and it gave me 205 intensity mins for a 10 mile walk. The breakdown was z4 2mins, z3 19 mins and zone 2 140 mins. Today I did the same mileage but a harder (steeper) walk and it recorded z4 4 mins, z3 64 mins and z2 113 mins. It gave me only 20 intensity mins!! Zone 3 & 4 were higher today but I only got 20 mins. The same has been true of a cardio workout - my vivosmart 5 would give 60 mins but the vivosmart 4 for exactly the same workout (dvd) gave 30 mins.