There’s a video out there in the ether that purports to measure time dilation in a car. I’ve already shown that this can, and does, happen, but you need to have some pretty expensive toys at your disposal to make the measurement.
For a good experiment (I’m perhaps charitably assuming this wasn’t just an out-and-out fraud), one would also want to measure the stopwatches against each other to make sure they were running at the same rate, and calibrate them if they weren’t. Ideally you’d want several clocks, but that’s a little advanced for this level of execution. Then, you’d want to make sure that you weren’t perturbing the clocks with different environments, like temperature differences, so make sure you aren’t blasting the AC on the stopwatch. Finally, you’d want to predict the difference to compare it to the measured difference. Ignoring effects from any elevation changes, a half-hour trip at 60 mph is going to give you a dilation of around 7 picoseconds.
My conclusion is that your stopwatches suck.
“For a good experiment (I’m perhaps charitably assuming this wasn’t just an out-and-out fraud), one would also want to measure the stopwatches against each other to make sure they were running at the same rate, and calibrate them if they weren’t.”
That’s part of how we can tell that the video is “intentionally” bunk, and not just some spurious result from bad equipment. At the end (roughly 1m30s into the video), he did PRECISELY that, and the times on both stopwatches matched.