Speed boots can give you additional movement points equal to their enchantment. There are a couple non-mutually-exclusive options for doing this:

• This only works for blessed speed boots.
• If the enchantment is negative, they subtract movement points, to a minimum of 1.
• To avoid the amount of movement points being predictable, the number of bonus movement points is slightly fuzzed by adding a random number from -1 to +1 each time it’s applied.
• The boots do not themselves provide any bonus movement, or provide a smaller one than they do now. The bonus from enchantment makes up the bulk of it.