"What the fuck is a kilometer" is a phrase often uttered by Americans who have never experienced the beauty of the metric system. Usually the phrase is enjoyed with eagles screeching in your ear and guns being fired. No matter how many times they were taught what a meter was in elementary school or how to convert between units, most Americans still don't understand. I think that part of the issue is that people don't want to do formulas on their head. In school, they may have needed a formula, but that doesn't apply to the real world. It's far easier to convert than they think it is.
It's simple: 100kph is 62.13712mph. While that initially might not help, 62.13712mph is quite close to 65mph, also known as highway speed. It is so much easier to think of kilometers per hour as "percent of highway speed" than anything else. Say a road's speed limit is 35mph. That's just over half of highway speed. Let's estimate that it's 55kph, or 55% of highway speed. The real answer is 56.3kph. That's plenty accurate for everyday use, especially driving.
Great, but what about miles and kilometers, not mph and kph. Well since you can just cancel out the hours from each side, 100km is 62.13712mi, or rather "a kilometer is about 65% of a mile"... which is awfully close to "two thirds of a mile". Two thirds of a mile is something that most Americans would have a better sense of than "0.6213712mi".
How about meters and feet? Well a yard is 3 feet, and a meter is basically a yard. You can pretty much just think of a meter as 3 feet. It's really not as daunting as most Americans think it is to convert, especially when you aren't required to be very precise.
Now it's time for me to do an about face. Celsius is not that great. Yeah, fight me. It's certainly simple to define, since 0ºC is the temperature that water freezes and 100ºC is the temperature that water boils, but that doesn't mean that it's easy for people to understand. Remember that this post is about everyday human understanding, not scientific usefulness. Nearly any time that humans want to know a temperature, it has to do with the temperature of air.
For that, Fahrenheit is perfect. Just as kph ends up being a good analogue for "percent of highway speed", Fahrenheit ends up being a good analogue for the range of human temperatures. 32ºF is the freezing point of water. Nobody cares about that... until you realize that snow begins 1 third of the way up the scale. Most comfortable indoor spaces are kept between 68 and 75 degrees, which is about two-thirds to three-quarters up the percent scale. 100ºF is a hot day, and it really doesn't get much higher than that.
The hottest temperature ever recorded was 134.1ºF, which is not too far outside the scale, and applying that to kph, 134.1kph is 83.3mph, which is also awfully close to the highest speed limit in the country, that being 85mph. And yes, that speed limit is found in Texas, because of course it is.
Now it's time for liters and gallons. By now, you get the formula. 1 liter is 0.264172 gallons. One rule of thumb later, and a liter is 25% of a gallon. But trust me, America already knows what a liter is. It's half of a 2L coke bottle.
In conclusion, Europeans should thank us for not making up our own unit of time. "one metric second is 0.583 US seconds" or some bs.