How to initiate the DLC missions in Saints Row 4

The Saints Row Wiki lists the cars in the game and sorts according to their speeds and categories.

From that source, the fastest cars are (info in parentheses are where/how to get them*):

• 2-person capacity cars
  • Attrazione
  • Wraith (Shaundi's "Under Pressure" side quest reward)
• 4-person capacity cars
  • Blade
  • Genkimobile (Kinzie’s "Gambit" side-quest reward)
  • Hammer
  • Infuego
  • Pacemaker
  • Peacemaker (driven by police)

If there is no unlock/reward info, then that means that the cars are either randomly parked or driven by NPCs. The 'unlockitall' cheat and the Additional Cheats mod (PC only) can also add many of those cars to your garage.

^{Source: Personal in-game experience; some info are from the cars' respective Saints Row Wiki articles.}

I did this on the Xbox earlier today. Does not require cheats, mods, special inputs, or beating the main story.

1. Start an on-foot Mayhem activity (like the one where you use the black-hole gun).

2. Stay on the road the activity's start-point is found on and wait for a tank to show up.

3. Fire a black-hole near it, but not close enough to destroy it, just to deal a bit of damage; just on the edge of the black-hole's suction or blast radius. This will kill the driver and disable the tank.

4. Once disabled, quit the activity, rush to the tank, hop in, and scan it.

Bam! You now have the Challenger.

NOTE: This may take a couple tries to get it right, but it does work, even if you completed the activity before attempting to get the tank. Took me three tries to get it, since the first time it got blown up by a friend, but we found a Bear APC and tested the trick with that -- it worked flawlessly -- and second time it spawned on the road behind the start-point, so after exiting the activity, it had already disappeared before I could reach it.