The red truck is the wild symbol. It can land on the middle three reels and substitute for all the other symbols. Furthermore, it doubles the payout when it helps to form a win.
The scatter symbol features an exhaust pipe and it offers scatter payouts of up to 100x the total bet for the spin.
The slot's main bonus feature is the Dollar Ball Progressive Jackpot. To play for the jackpot you need to enable the game and then pick five numbers between one and forty-nine. With each spin of the reels a lottery style draw takes place and you will win prizes based upon how many numbers you match. Matching one, two or three numbers awards a fixed payout of 1x, 2x or 25x respectively, matching four of the numbers awards 1% of the progressive jackpot and matching five numbers awards the entire jackpot.
Highway Kings offers a huge top payout of 10,000x if you land five red truck symbols along a payline. The yellow truck can payout up to 5,000x, the green truck and petrol canister can both payout up to 1,000x, the petrol pump and spark plug can payout up to 300x, the dice and piston can payout up to 200x and the wheels can payout up to 50x.