I bought a Tesla Model X in 2023 and kept staring at the 17-inch display sitting idle while I worked on my 14-inch MacBook. I often drive to a quiet spot and work from the car, so I wanted to use that screen as a second monitor.

Nothing existed to do this, so I tried building it myself. It took over a year of failed attempts:

- Hardware mod: didn't want to void the warranty - Zoom screen share: worked but terrible latency, no fullscreen - HLS streaming via OBS: ~2 second delay, unusable as a monitor - WebRTC: Tesla's browser didn't support it at the time

A few Tesla software updates later, I tried WebRTC again and it worked. But I still needed a pile of hardware: an LTE router, an OpenWrt router, and a dummy HDMI adapter. The setup was ridiculous.

Then I realized every piece of hardware had a software replacement:

  LTE router -> iPhone USB tethering
  OpenWrt router -> macOS Internet Sharing (bridge100 + bootpd + NAT)
  Dummy HDMI -> Apple's CGVirtualDisplay API

Total extra hardware cost: $0. Just a MacBook and an iPhone.

One interesting discovery along the way: Tesla's browser blocks all RFC 1918 private IP ranges (10.x.x.x, 172.16.x.x, 192.168.x.x). My guess is it's a security measure to protect the car's internal network. The workaround was configuring the bridge interface to assign public IP addresses instead.

I packaged everything into a macOS app called SideDisplay. Free trial, 30 min per session, no sign up. If you have a Tesla and a Mac, try it and tell me what's broken.

I also wrote a detailed development story covering every failed attempt and the technical decisions behind them. It would mean a lot if you checked it out: https://sidedisplay.co/story