In Australia, we use the metric system. I'm not aware of anyone who isn't aware that a mile is 1.6km, a foot is 30cm, an inch is 2.5cm, and any other unit of length is unnecessary, particularly in a game. In other words, we can convert from one to the other instantly, without issue.
Weights? They're annoying. 2.2lbs to a kg is OK, but stones and other imperial measures of weight just aren't commonly used.
Temperature? F or C is doesn't really matter. People used to C don't commonly convert from or to F. C is a much easier scale when you look at using other metric units.
Everything else? Pressure, area, etc? A zillions times easier in metric so most people don't do the conversions instantly or easily (or at all).
For a game system, though, distance, weight, and temperature are the key ones. You could easily provide temperature in both C and F and then just run distance and weight in imperial and anyone using metric wouldn't have any issues. If you must use things like leagues and and so (even fathoms), spell them out somewhere for both imperial and metric equivalents, but I'd generally avoid them unless you want to make the point that a particular culture is different (in which case you could just make up anything you like).
In short, people who use metric every day have so much exposure to US and British media and culture it's not an issue for the main stuff.
Weights? They're annoying. 2.2lbs to a kg is OK, but stones and other imperial measures of weight just aren't commonly used.
Temperature? F or C is doesn't really matter. People used to C don't commonly convert from or to F. C is a much easier scale when you look at using other metric units.
Everything else? Pressure, area, etc? A zillions times easier in metric so most people don't do the conversions instantly or easily (or at all).
For a game system, though, distance, weight, and temperature are the key ones. You could easily provide temperature in both C and F and then just run distance and weight in imperial and anyone using metric wouldn't have any issues. If you must use things like leagues and and so (even fathoms), spell them out somewhere for both imperial and metric equivalents, but I'd generally avoid them unless you want to make the point that a particular culture is different (in which case you could just make up anything you like).
In short, people who use metric every day have so much exposure to US and British media and culture it's not an issue for the main stuff.