Sponge

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

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.

In terms of their hidden stats, the Sponge tires excel very poorly while underwater, but their air stats are given a strong boost to make up for it.

Mario Kart 8 / Mario Kart 8 Deluxe
The Sponge tires also appear in Mario Kart 8, where their appearance has been altered to an extent. Their stats have also been modified a bit in that they don't boost acceleration as much, and they're not as light as before. Their handling does get hindered to boost traction, however.

Much like in Mario Kart 7, the Sponge tires are among the worst when it comes to underwater performance, but they do get a mini-turbo boost to make up for it. The Sponge tires share their stats with the Wood and Cushion tires.

In Mario Kart 8 Deluxe, the Sponge tires are available from the start.

Stat boosts
Stat values are out of 6.

Mario Kart 7

 * Speed: -0.25
 * Sea Speed: -1
 * Air Speed: +1
 * Acceleration: +0.5
 * Weight: -0.75
 * Handling: 0
 * Sea Handling: -1.25
 * Air Handling: +1
 * Off-Road: 0
 * Mini-Turbo: 0
 * Stability: 0
 * Drift: +1

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: +0.25