Sponge

The Sponge is a set of unlockable tires in Mario Kart 7 and Mario Kart 8.

Mario Kart 7
The Sponge tires offer an acceleration boost that's tied with the Wood tires, but they also hinder speed and weight. In fact, the Sponge tires offer the second lowest weight boost, losing only to the Roller tires.

Mario Kart 8
The Sponge tires also appear in Mario Kart 8, where they have identical stats to the Cushion and Wood tires'. Their appearance has also been altered to an extent.

Stat boosts
Stat values are out of 6.

Mario Kart 7

 * Speed: -0.25
 * Acceleration: +0.5
 * Weight: -0.75
 * Handling: 0
 * Off-Road: 0

Mario Kart 8

 * Ground Speed: -0.25
 * Water Speed: -1
 * Air Speed: +0.25
 * Anti-Gravity Speed: -0.25
 * Acceleration: +0.25
 * Weight: -0.25
 * Ground Handling: -0.25
 * Water Handling: -0.5
 * Air Handling: 0
 * Anti-Gravity Handling: -0.25
 * Traction: +0.5
 * Mini-Turbo: +0.75