What is the best time to expand a mobile car wash business? If everything were perfect and you could choose the best time to buy a new mobile car wash rig, you should buy it in April and have it running by the 20th of May. If you have 1.5 times the business of your first unit, then you automatically have enough business for one-half of a second truck, van or trailer unit combination. In June, the seasonal increase would take care of the rest. That is the simplest way. If you buy an additiona.