If you are on low end PC and can live with lower res mirrors - Go to the data/ vehicles/ car specific folder - Find MIRRORS.BMP document and resample original 4096x510 bmp to 1024x103 or even 512x52 px - Duplicate MIRRORS.BMP to the main game data/ vehicles folder Backup your MIRRORS.BMP first!
That's a nice tip, I've always use it. Helps a lot with FPS issues. But there's always a corner or two (especially harpins) where FPS could still drop a little. But now.... since I've started to use the amazing Crew Chief spotter, I've disabled rearview mirros completely. The in-game spotter also works just fine. I keep track of rear car looking their time gap, and when he tries a move, the spotter tells me if he's right or left of me. Of course, that's fine in singleplayer, since AI is very predictable and hardly tries a passing move. In multiplayer though, your MIRRORS.BMP tip rocks. This MIRRORS.BMP file does not exist in 'Vehicles' folder. But you can create it in PaintBrush! Just open it, set size as @Renoista said in OP, paint it all black, and save it as MIRROS.BMP inside "Automobilista\GameData\Vehicles" folder. Regards!
I thought that older users already know about this but could only find tip about enlarging bmp for drivers wishing ultra res mirrors. Especially when driving formula I noticed frame drops and after messing with settings discovered how much effect mirrors have on FPS. It would be cool if Reiza add options for low(1024) high(4096) and ultra(8192px) mirrors. Another tip if You really need extra fps is to edit yourname.PLR file inside UserData and change in Sound options "Maximum Effects" to 12 or 10. PLR file can be sensitive to editing so please Backup first!
Extra FPS is always welcome. Another tip: disable replay saving in PLR. Record Replays="0" // whether to record replays or not The replay will be available to you while you're in the garage though.