You start the game with a complete car. It isn't great, but it runs. And then you have some choices to make about whether to take it out and race it, or stay in the garage and keep trying to improve it. (Also there's a slightly apocalyptic/zombie mechanism, to keep things moving. Call it what you will.)
1. Separate all 18 of the "1" cards. Put the six pink ones aside; you will not use them. Give each player one full set of "car parts" cards valued at 1. This is the beat-up pile of junk that you start with, but it runs. 2. Shuffle the remaining cards valued 2-6 and place them face-down in a draw pile. 3. Give each player one full set of car-part tiles and arrange them, symbols up, in front of each player. 4. Lay out 12 face down tiles face down in a track. Place the two cubes (one "car" for each player) on the fourth tile from one end. They'll be drag racing to the far end. 5. Deal three additional cards face down to each player. Each player then selects six total cards of the nine he holds to make up his car, and "installs" them by placing them face down adjacent to the corresponding tiles in front of him. The remaining three cards stay in his hand. A player can always look at the cards that compose his own car.
1. Each turn a player may either (a) work on his car, or (b) challenge his opponent to a race. 2. Working on the Car - Draw 3 cards, either from anywhere in the scrap heap (see below) and/or the top of the draw deck, in any combination. Replace any number of your installed car parts from your hand. Discard down to 3 and put the discards in the scrap heap. 3. Challenging to a Race - Roll the die. The number rolled represents the car part that will determine who wins the race. So if the challenger rolls a "5," the outcome of the race is going to depend on who has better tires. The challenger is stuck with the number he rolls. The player receiving the challenge can either accept it and race, or "chicken out." 3A. Chicken Out - The challenged player declines to race, and allows the challenger to draw one card at random from his hand. The challenger then proceeds with his turn as normal, drawing 3 (for a hand total now of 7), swapping parts, and discarding down to 3. Play then passes to the player who was challenged, who draws 3 for a hand total of 5, swaps, and discards to 3. 3B. Race! - Both players turn over the part card indicated by the die roll (then put it face down again). The player with the higher value moves his card forward by the difference between the two. So a player with "5" tires wins over "3" tires, and moves forward two tiles on the drag strip. In the event of a tie, no players moves. If a race is accepted, regardless of outcome, the player who issued the challenge does not draw or swap parts, and play passes to the next player. 4. End of Turn - Every round, after both players have taken a turn, remove one tile from the end of the track behind the cars. Imagine this to be an approaching swarm of zombies, or the earth's crust collapsing into a chasm, what-have-you. It's just something designed to get the gearheads to stop tinkering endlessly and DRIVE.
The game ends when one player drives forward off the last tile in the track (winner, kisses his best girl), OR, when one player is on the rearmost tile in the track when it is removed (loser, dies horribly). If both players are on the rearmost tile when it is removed, you can either bump them both up one, or consider them both losers--just house-rule that however you want.
The length of the track and starting position might need to be refined. If the track is too long the players work through the whole deck and everyone is driving 5's and 6's. If it's too short, one good race (6 v. 1) can all but end the game. I see no reason this variant couldn't work with three players, but the length of track may need to be different. Beyond 3 players and you'd run out of "1" cards to start with (unless you play with two GBOG sets).
The variant preserves the opportunity noted above to watch your opponent's hands closely, to see what he replaces, discards, or keeps. There's also some opportunity for subterfuge, "false swaps," and misleading discards.
Opting to race earlier is riskier, in that your car is undeveloped, but could pay off with a lucky roll (your opponent's car is undeveloped as well). Also, having that seventh card in your hand can make a big difference early in the race when you have a lot more room for improvement.
The natural tendency will be to discard the lowest possible values to the scrap heap (to keep them from being picked up for free), but the "chicken out" mechanism provides a counter-incentive not to hold too many rich parts in your hand.
Drawing from the top of the deck every time in hopes of pulling a 6 can be tempting. But given the short track, there can be real value to upgrading a "1" value slot to a 2 or a 3 from the scrap heap, if only to keep a losing race from being an absolute blow-out.