I think most vehicles with 6 gears are geared around the same general minimum shift points to avoid engine lugging. For slow acceleration that would be approx. 20,30,40,50, and 60 MPH (32, 48, 64, 80, and 97 km/h). Ever vehicle I've owned, regardless of mass and engine size, has struggled to some extent below those points. This makes sense since the intended purpose is to get up to freeway speeds and then, if not going even faster, to cruise along at relatively low and non-annoying RPMs.