How is finding the unit rate before graphing a proportional relationship

One example, take the conversion fact that there are 5 miles for every 8 km.

Now if we want to work out how many kilometres in 6 miles, the conversion above is not going to help straight away.

We first need to find how many kilometres in ONE mile [which is the unit rate]

5 miles = 8 kilometres
1 mile = 8/5 kilometres
1 mile = 1.6 kilometres

Once we know the unit rate, we can then work out the number of kilometres for any number of miles.

For example:
6 miles = 6×1.6 = 9.6 kilometres
25.5 miles = 25.5 × 1.6 = 40.8 kilometres

This does not make sense