There have been many top-down racers inspired by games like Super Offroad or Micro Machines for the iPhone, but so far the genre is fairly barren on the iPad. Racing Mania [99¢ / $3.99 HD] changes all that, bringing a real top-down racer with car upgrades, local multiplayer, 15 different tracks, and 10 cars to both devices.
On the iPhone, Racing Mania isn't much to write home about. There are other games (such as the recently released LilRacerz [99¢]) that offer more variety and a better racing feel. However, on the iPad, the claustrophobic viewpoint of Racing Mania (and other similar games) is pulled far back allowing you to see an amazing amount of the track at once which really changes the whole feel of the game.
iPhone left, iPad right.
On the iPhone, once you purchase a few top speed upgrades for your car, successfully navigating around the tracks seems to require more memorization than anything else as you will be going too fast to really be able to prepare for upcoming turns without bouncing off of them otherwise. On the iPad, you have enough screen real estate to plan out really precise racing lines and burn around the track with ease.
Racing Mania is hard to recommend for the iPhone, but may be worth checking out just for a taste of how well games like this work on the iPad. The upgrade system is a little basic, and the controls aren't ideal, but you don't have to play Racing Mania HD for long as all to realize the potential the genre has on the iPad.