The Boost Gauge is presented as an energy gauge of varying design which appears on the heads-up-display in the games. For as long as there is energy remaining in the Boost Gauge, the player can perform the Sonic Boost/Fire Boost. Using these techniques drains the Boost Gauge though, and should it run out, the player can no longer perform them. The means of charging the Boost Gauge vary between games.
The Sonic Rush series introduced a specific reoccurring mechanic for the Boost Gauge, where it is dubbed the "Tension Gauge." When the Boost Gauge is charged to maximum capacity, it gains unlimited energy for a short time, allowing infinite use of the Sonic Boost/Fire Boost during that time.
Sonic Rush series
In Sonic Rush and its sequel, Sonic Rush Adventure, the Boost Gauge is referred to as the Tension Gauge. It also uses the Tension Gauge mechanics, allowing unlimited energy for a brief time when overcharged. In these games, the Boost Gauge is presented as a gauge on the left side of the screen and is made of several stabled segments with a star icon on top. Their designs differ between games though: In Sonic Rush its segments are flat, and in Sonic Rush Adventure the segments are arrow-shaped while the gauge has more elegant details.
The Tension Gauge can be charged to a maximum of 300%. The gauge's color turns from blue to orange, and then red in accordance to the amount of energy it it. When at maximum capacity, the Boost Gauge will flash and its star icon spins.
In these games, the Boost Gauge is filled at 100% capacity at the beginning of each Act and is used to power both Super Boost and Fire Boost. The player can replenish it by performing Trick Actions, defeating enemies, or obtaining Tension Bonus or Max Tension Bonus power-ups. Conversely, the Boost Gauge will deplete when the player takes damage.
In Sonic Unleashed, the Boost Gauge is referred to as the Ring Energy Gauge on the Xbox 360/PlayStation 3 version and the Boost Gauge on the Wii/PlayStation 2 version. All versions display the gauge in the lower left corner of the screen. On the Xbox 360/PlayStation version, the Boost Gauge is a dark gauge with a red-yellow bar within it, a Ring icon and counter on the left, and an overarching multicolored bar for measuring speed. On the Wii/PlayStation 2 version, the Boost Gauge is a Ring icon and counter with horizontally-aligned bar units next to it.
In these games, the player starts the Stages with no energy in the Boost Gauge. The player can refill the Boost Gauge by collecting Rings, using the Sonic Drift, or performing Action Chains (Wii/PlayStaion 2 version only). Taking any damage still depletes the gauge though. On the Wii/PlayStation 2 in particular, using the Sonic Boost costs one unit from the Boost Gauge.
The player can increase the Boost Gauge's size, depending on the version of the game. On the Xbox 360/PlayStation 3 version, the player can increase Sonic's Ring Energy parameter to enlarge the gauge permanently up to six times. On the Wii/PlayStation 2 version, the Boost Gauge can be increased temporarily by one unit for every thirtieth Ring (up to six units). Conversely, the maximum number of units will decrease by one when the player takes damage (down to three units).
In Sonic Colors, the Boost Gauge is merged with the Wisp Gauge as the Boost is classified as a Color Power in this game. Nevertheless, it is still referred to as the Boost Gauge. On the Wii version, the Boost Gauge is a horizontal blue bar in the lower left corner of the screen with a stylized circulator on the left that displays the Wisp Icon. In the Nintendo DS, the Boost Gauge is positioned on the left side of the screen and looks like it did in Sonic Rush Adventure, but with a blue bar on the left side and the Wisp Icon on top.
The Boost Gauge is similar in structure to the Boost Gauge in the Sonic Rush series, capable of being charged to a maximum of 300% and the color of the gauge changing from blue to orange, and then red to indicate the amount of energy in it, although it lacks the Tension Gauge mechanic. When beginning an Act, the Boost Gauge will be empty from the start on the Wii version, but on the DS version it will be filled at 100% capacity. The player can replenish the Boost Gauge by collecting White Wisps or defeating enemies holding White Wisps.
After collecting all Special Rings on the Nintendo DS version, the player is awarded with an option that grants the Boost Gauge unlimited energy.
In Sonic Generations, the Boost Gauge is back to its original form and singular function. On the console/PC version, the Boost Gauge is blue with a green energy bar, a thick left end, and a stylized white and yellow frame. On the Nintendo 3DS, the Boost Gauge is similar to the console/PC version's, except that its shape is mirrored, its color is cyan, and it lacks a frame.
The Boost Gauge only appears in Act 2 of all the Stages as it is only there the Boost is usable. On the Nintendo 3DS version, it possesses the Tension Gauge mechanic, allowing for brief infinite energy when overcharged. The player can replenish the Boost Gauge by collecting Rings, defeating enemies and performing Freestyle Tricks. Should the Boost Gauge be full on the console/PC version, performing Freestyle Tricks will charge the Boost Gauge with extra energy, which is showcased by the energy bar extending outside its frame.
On the console/PC version of the game, there are various Skills that allows manipulation of the Boost Gauge, including the Auto-Gauge, Endless Boost, Gauge Rebooter, and Boost Gauge Up.