Both kilometers per hour and miles per hour (mph) are units to measure speed. A measurement of average speed takes the distance traveled and divides it by the time it took to travel that distance. Because both speeds give distance per hour, the only units that needs to be changed are kilometers to miles, a conversion which can be done in under a minute.
- Know the conversion rate
There are 1.609 kilometers in every 1 mile. If 1.609 kilometers = 1 mile, then 1.609 kilometers/1 hour = 1 mile/1 hour because performing the same operation to both sides of an equation keeps the equation balanced. The conversion rate can also be written as 1 kilometer = 0.621 mile because one kilometer is just over three-fifths of a mile.
- Cancel out the kilometers
If you are converting kilometers to miles, you need to cancel kilometers. Multiply a known number of kilometers by a conversion fraction with kilometers in the denominator. Because 1 kilometer = 0.621 mile, the fraction 0.621 mile/1 kilometer can be used. Multiply 90 kilometer/hour by the conversion fraction: 90 kilometer.hour x 0.621 miles/kilometer = 55.89 miles/hour.
- Check your answer
Check that your answer makes sense. There should be between three-fifths and two-thirds as many miles are there are kilometers. For example, if you started with 90 kilometer/hour, the equivalent in miles per hour should be between 54 (three-fifths of 90) and 60 (two-thirds of 90).