Refers to concrete mixer trucks which up until recently could haul only nine yards due to highway weight restrictions. Depending on your pour, you could order a partial load, or ask for "the whole nine yards."
Um---It may have been used in that context at some time, but the origin of "The whole nine yards" is fighter planes, WWII ish.
Belted 50 cal ammo came in belts 9 ft long. 3 belts--27 ft.--9 yds. was
a standard load per gun.
Shooting off "the whole nine yards" refered to a trigger happy/nervous pilot
who---instead of firing controlled bursts---held the trigger down and
ran the guns dry.