Hey so far I haven't seen this anywhere on the internet so I am going to post my findings.
If your like me on GTA IV you use cheats
Well I usually use the spawn car cheats because I like the Super GT.
Well you probably know you can spawn one car and if you spawn another one the first you spawned disappears and is replaced by the newly spawned car. Well by playing around I found the if you spawn a car get in and go back to the cheat menu and choose to spawn another car you will have two spawned cars.
Car Spawn Glitch
Linear Mode
