Asimilarity ratio is the ratio of the lengths

Similarity ratio is a ratio of two figures having the same side.

Ratio can be rate but rate can never be ratio. In essence, rate is comparison between ratios.  While ratio is comparison between two or more numbers. Further, ratio on one hand, involves numbers either in amount, size, measurement, degrees, percentages or fractions with the absence of specific unit of measurement. On the contrary, rate is comparing quantities, amounts or unit of events happened expressed in a specific measurement or expressed under time. Take for instance, an example, Joe eats 2 while John eats 4 meals in a day. The ratio can be Joe: John, 2:4 meals. While the rate, is Joe eats 2 meals/day and John 4 meals/day.