From the Super Mario Wiki
The Gold Tires (known as the Gold Wheels in Europe and Australia) are a special set of unlockable tires in Mario Kart 7 and Mario Kart 8. They offer the exact same stat boosts as the Slick tires, except for that they give up some more acceleration to enhance weight. The Gold Tires are unlocked by collecting 15,000 coins, or by getting a star rank on every GP cup for each engine class in Mario Kart 7. In Mario Kart 8 they can only be unlocked by beating every Staff Ghost in Time Trial mode. Like the Slick tires, the Gold Tires offer the best speed boost, but they also offer the worst off-road boost. Additionally, they're tied with the Slick, Monster, and Red Monster, for the weakest handling boost in the game.
When the Gold Tires are combined with the Gold Standard body and Gold Glider, they complete the whole Gold Standard. This kart gives great speed (the 2nd biggest boost in the game) and good weight, but has a bad handling boost, and abysmal off-road (the smallest boost in the game, which is on par to if the Slick wheels were used).
The Gold Tires also appear in Mario Kart 8 with the other gold parts from Mario Kart 7. They look slightly different than they do in Mario Kart 7, and their stat boosts/drops are different, being much more balanced compared to the ones from Mario Kart 7.
Stat values are out of 6.
Mario Kart 7
Whole Gold Standard
Mario Kart 8
Names in other languages