It's a ratio problem, just like I learned in 4th grade math. It became useful and important in the 6th grade when I started building go-carts in my back yard. I had to understand how fast I could go with a 2 HP lawn mower motor...
21 divided by 8 = 2.625:1
20 divided by 8 = 2.5:1
19 divided by 8 = 2.375:1
18 divided by 8 = 2.25:1
17 divided by 8 = 2.125:1
21 divided by 7 =3.00:1
20 divided by 7 = 2.85:1
19 divided by 7= 2.71:1
18 divided by 7= 2.57:1
17 divided by 7= 2:24:1
The higher the resulting end number, the greater reduction in the speed of the output.
See the attachment for help your speedo gear selection.
For those of us too busy to do the math, see LRS speedo gear calculator
http://www.latemodelrestoration.com/products/Mustang-Speedometer-Gear-Calculator