Searching a list of pairs of numbers
Maybe you can use dict instead of str to store the driver-load values. If the driver is the key, you can search easily.

#!/usr/bin/env py

s = {'5.9':'1', '7':'1.5', '10':'2', '18':'4', '40':'8'}
load_value = raw_input("Enter a load value\n")
driver = s.get(load_value)
if driver is None:
    print "Could not find driver value"
else:
    print driver