My first car had a broken speedometer. It was a "hand" style like a clock. Instead of moving to the speed, it would spin 365 degrees. The faster the car went, the faster it would spin. Turns out, I acclimated to it and generally knew what speed i was going (relatively speaking).

The lesson, I think, is everything is relative. Even a dashboard with flawed data that is "consistent" can highlight anomalies. And often, that's all you really need out of them. (Or the lack of anomaly)