Dish soap for a homemade car wash solution is actually best suited for the task of washing your automobile when mixed with laundry detergent. Dish soap can be used on it’s own, as can laundry detergent, but together they combine to make a car wash solution that will effectively remove grime as well as just dirt. A good ratio to use is one bucket of water, one cup of dish washing soap, and 12 tablespoons of powdered laundry detergent. Additionally, it’s best to add borax to the solution to reduce streaking. It is also important to extensively rinse your vehicle after washing for the same reason.
For more info, read How to Make Your Own Car Wash Soap