This is pretty normal since the track affects your gearing. If
you are going uphill, you'll need to drag a little bit longer at
the lower gears to build up power before you can shift to a higher
gear. If you are going downhill, it'll be the opposite. If you use
Iro (last track) as an example, when going uphill or downhill, you
can see the difference in gearing.
When you hear the engine giving out a high pitch whine, that's the
signal to change to a higher gear. But you should do it before this
high pitch whine.
