"I was doing 70 down the highway" is absolutely the sort of thing people say. From context (this conversation takes place in the US, most highways have a speed limit somewhere around 60-70mph) people understand that you are talking about miles per hour.

And yet, on the internet without the extra context, I can't tell if you're going slowly in km/h or the usual in mph. With dB that depends on whether you're taking to a person familiar with the context-specific conventions or not.