Distance: One kilometer (km) = 0.62137 mile.
Estimate  by dividing kilometers by 10 and multiplying by 6.
(Or multiply by 0.6; or take 5/8 if that works easier with a given distance.)
 

Speed: Speed in kilometers per hour (km/h) = 0.62137 mph.
Estimate  by dividing kilometers by 10 and multiplying by 6.
 

Area: One square meter = 10.7639 square feet.
Estimate  by multiplying by 10 or 11.
One hectare = 2.471 acres. Multiply hectares by 2.5 and you'll be close.
  

Fuel Consumption: In Europe most often given in liters per 100 kilometers.
Estimate by multiplying the liters per 100 km figure by 0.4, and divide 100 by the result.
The mpg result will be about 6% high.
 

Height (Altitude): One meter (m) = 3.28083 feet.
Estimate  by multiplying meters by 3 and adding 10%.
 

Length: See Height for meters and feet. A centimeter is about 3/8 inch.
2.54 cm make an inch. Four inches is the equivalent of 10 cm; 30 cm per foot.
Estimate inches by 10*cm/4, or divide by 0.4, or multiply by 2.5.
 

Weight: One kilogram = 2.20462 pounds
Estimate  by multiplying by 2 and adding another 10%.
One gram = 0.03257 ounce, about 28 in an ounce.
Estimate grams/ounce at 25 or 30 for convenience.
 

Volume: One liter = 1.0567 quarts or 0.26417 Gallons.
Estimate  1:1 for quarts or 1:4 for gallons and add a smidgen (~5%).
 

Temperature: ^oF=^oC*9/5+32.
-- Estimate  Fahrenheit at twice Celsius plus 30. Or ...
0 ^oC = freezing; 10 ^oC = quite chilly; 20 ^oC = cool room temperature;
30 ^oC = quite warm; 40 ^oC = you and your camel have never been so hot.