I've found an app that's pretty useful for timing cars, and no it's not just a stopwatch. The Seconds Count App was developed for athletes and racer as a way to use video to time events. The way it works is pretty simple. Record a video of the race, scroll through the footage and add a START and FINISH marker, and the app tells you the time in-between those 2 points. It's really a pretty simple idea but it took me a lot of searching to find an app that could do this.
There's a Lite (FREE) and Pro version. I believe the pro is an anual subscription of $5.99 per year, which is a great price. The main advantage of the pro version is you can do video timing at higher frame rates (60-120fps if your phone supports that) giving you a more accurate result. There's also a feature to store lap and finish times in a table. The app is available in both the Apple App and Google Play stores.
Overall, if you have a decent smart phone, it's a great app to get some actual times on your cars.
Also a little tip I found is to make your start or finish line stand out so you can easily see the cars crossing when you review the footage.