What is the 'Boost' gauge used for?

Solution 1:

I emailed Fingersoft support and they replied with this:

The boost gauge indicates the use of turbo on vehicle that have it.

Sami-jukka Piippo (Fingersoft)

This means the boost gauge is different for each vehicle, simply displaying the level of turbo on that particular vehicle. For example the tank has a high boost (or turbo) according to the Hill Climb Wiki, making the boost gauge max out at a high position. This is shown in this picture:

There is no boost that you can activate, nor does it give you an advantage when full. The boost gauge is accurate when at full RPM. The top boost can be affected by upgrades.