It's pretty straight forward. You need to complete all of the events on City Rumble beeing the 1st place.

There are four events in total

Quick Race - 52 events
Time Attack - 32 events
Boss Race - 5 events
Secrets uncovered - 9 events (here's a link to another guide which shows you how to uncover them: https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2616706194)

Winning all of the 52 Quick Races will give you the Duck N' Roll.

Winning all the 32 Time Attacks will give you the Hi-Po Mauler.

Beating all 5 bosses on Boss Races will give you the Ramblin Wrecker™.

Discover and winning all of 9 secrets events will give you the Oldsmobile 442.

When you win all the events (and, consequently, all the secret cars) you'll be awarded with the Volkswagen Beach Bomb™!

In real life, this is considered the rarest and most expensive Hot Wheels. It have a surf board on the rear window, which caused this version to never make to mass production, 'cause it couldn't fit rightly on the Hot Wheels Super-Charger Race Set tracks.

Also, when you complete all the events you'll be awarded with the achievement "I Want 'Em All".

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