The dash cam would work. I wouldn’t trust obd because they could be sending the same info through or doing some VW diesel gate stuff. Maybe comparing obd to waze to what’s displayed on screen would be better. When I mount different size tires on my vehicles I use waze to compare the speed on the speedometer vs waze. Most vehicles in the past read faster than it was going, it’s only in the past few years I’ve seen them being more accurate, around when telemetry started being more prevalent.