Important for car (if you rent one, it will automatically have mph, but it is still useful how fast you are going for e.g. braking distance):
All values are rounded for convenience; the most frequent values are
30 mph = 50 km/h
55 mph = 90 km/h
65 mph = 105 km/h
Every car has cruise control. If you want to explore the USA by car, I recommend strongly to use it because the long distances together with the consistent speed limit and no action (In Europe you are overtaking, adjusting the speed because the speed limit changes, changing the lane and leave the highway) causes you to violate the speed limit easily and the punishment is prohibitive for European standards.
Fahrenheit in Celsius: Subtract 30 and halve the value
30 - 40 °F = 0 - 5°C cold
40 - 50 °F = 5 - 10°C cool
50 - 60 °F = 10 - 15°C fresh
60 - 70 °F = 15 - 20°C coolish
70 - 80 °F = 20 - 25°C normal
80 - 90 °F = 25 - 30°C warm
1 inch = 2,5 cm (width of a thumb)
1 foot = 12 inches = 30 cm (a big foot, multiply with 3/10)
1 yard = 3 foot = 90 cm (a bit shorter than a m)
1 mile = 1760 yards = 1,6 km ( multiply with 5/8)
The 24h system is not used widely in the US, they are (except the
military) still using the a.m. (before midday) and p.m (after midday) system. Everything which is not 12:XX is easy (1:29 p.m. = 13:29, 5:30 a.m. = 5:30), but the 12:XX are tricky because they are changing the a.m./p.m. during midday and midnight. Example: From one minute to midday it is 11:59 a.m., 12:00 p.m., 12:01 p.m......1:00 p.m. and at midnight the change to 12:00 a.m. occurs.
It is good to know that the calendar begins with Sunday (in contrast to Monday in Europe) and dates are expressed in the month/day/year format.
Drinking and car filling:
The US and UK pint and quarts are different, I forgot that
1 fluid ounce = 30 ml (US) used for small portions
1 pint ~ 470 ml (US) / 550 ml (UK) (half a liter)
1 quart ~ 900 ml (US) / 1,1 l (UK) (nearly a liter)
1 gallon = 3,8 l (important for filling the car and yes, petrol/gas is very cheap).
1 psi (pounds/inch²) = 1/15 bar = 7 kPa (1 bar = 100 kPa ~ atmospheric pressure)
15 psi = 1 bar = 100 kPa
30 psi = 2 bar = 200 kPa
1 pound = 450 g (1 kg = 2.2 lb)
1 ton = 1 ton
The floors are numerated with 1/G, 2, 3, 4...instead of the European G, 1, 2, 3...