Grand Prix Story/Cars

For unlocking cars there are 3 roads

1st- For the super car, A-class
 * Upgrade 20% roadster for Buggy
 * Upgrade 50% Buggy for Proto
 * Upgrade 50% Proto for Super-Car

2nd- For Spiral car, B-Class
 * Win Snowville GP for Dragster
 * Upgrade 50% Dragster for Cigar car
 * Upgrade 50% Cigar for Spiral car

3rd- For Aero car, B-Class
 * Win Japan GP for Wing
 * Upgrade 50% Wing for Sonic
 * Upgrade 50% Sonic for Aero car

Upgrading BOTH Aero car and Spiral car will give you the Mech car, S-Class.

You should only sell the cars either when you haven't used them at all on a race and are not useful and when you are really low on money. scrap for research data instead.