Sonic Riders: Zero Gravity

In story mode, the missions about collection a certain amount of rings count toward your overall ring amount.
being only one lap and 3 ring cap upgrades, you can get tons of rings quickly.

I've been performing this trick for quite some time now. Choose normal race and go to Gigan Device (the first level of the Babylon story.) When choosing your racer, (Shadow for me, but that's just my opinion,) select the Gambler gear. Then, stick to the front of the pack and to the higher path, and if you get every Ring CAP upgrade, fill all 250 rings and come in first place, you get 500 rings. Even if you don't get all 250, if you come in first, whatever rings you have will double. Oh, and try to get the most expensive gears first. That will save you some time. (Mind you, you may wish to mute you TV, as the music will annoy you all to hell after the first 5 races.)