Definition 1

Ratio
A ratio is used to compare two quantities that are measured in the same units, and a ratio has no units. 
The ratio of a to b is written as a : b, or a/b, where b ≠ 0. 
For example, the ratio of 40 cm: 1.5 m will be 40 cm: 150 cm which = 4 : 15.

Definition 2

Rate
A ratio of two quantities that have different units is called a rate. 
For example, if a worker is paid $30 for 6 hours of work, then his rate of pay is $30/6h = $5/h. 

Average Speed
Speed is the rate of distance travelled per unit of time. If it is unchanged over a period of time, it is called constant speed. But since the speed of an object is usually not uniform throughout a journey, the average speed is calculated. 
The formula to calculate average speed is = Total distance travelled/total time taken.