So, we've got:
1. Speeds (mph) and fuel (mpg in an X gallon tank)
2. Lumber (2x4)
Let's add a few more:
3. Milk (pints)
4. Beer (pints)
5. Ingredients in menus (pounds and ounces)
6. Human weight (stone, pounds and ounces)
7. Human height (feet and inches)
8. Vehicle heights for bridge clearances etc. (feet and inches)
9. Time (hours, minutes and seconds)
10. Date (days, months, years)
And that was just stream of consciousness, without a pause to think of other examples.
Seriously, standards are great. They help us to communicate unambiguously. And we have standards in the UK, and they are what I just listed. No-one here goes to the supermarket to buy 227g of cheese and 1.14L of milk. No-one goes to a car showroom and asks whether the fuel economy around town is better than 7.84L/100km, and most people's instinct would be that a higher number was better even if they had that reference point. A few people might describe their height in metres, but most people would say something like "five foot nine".
For projects where international collaboration is required, sure, agree a standard up-front, and it might as well be SI. Likewise for scientific and engineering applications, everyone is a professional and can agree to use SI. But for day to day life? You'd better hope someone going to a supermarket or a pub knows the same units as everyone else, because asking for 0.28L of beer at a crowded bar isn't going to make you any friends.