As a DirectX 11-game, DiRT 4 offers support for multiple graphics cards in the traditional way. The scaling of CrossFire is great, although unfortunately it is lower at the resolution where you need the performance the most (4K ultra). This means that this setting is still not really playable.
|framerate in fps||RX 580||RX 580 CF||Difference|
|Full HD medium||183,4||241,7||32%|
|Full HD ultra||76,7||132,4||73%|
|Ultra HD medium||63,1||117,8||87%|
|Ultra HD ultra||29,3||46,3||58%|
Nvidia does not offer support for SLI in DiRT 4 and an extra GPU only costs performance. As Nvidia-user you are better off not using SLI in this game at all.
|framerate in fps||GTX 1080 Ti||GTX 1080 Ti SLI||Difference|
|Full HD medium||290,7||285,7||-2%|
|Full HD ultra||139||116,3||-16%|
|Ultra HD medium||167,7||145,3||-13%|
|Ultra HD ultra||67,2||59,7||-11%|