Always wash the car on the grass
If you are not on water restrictions and want to wash the car, wash it on the lawn. It will give the lawn some water and the grass will use the detergent as a fertiliser. When you have finished with your soapy bucket, pour the dirty soapy water on the lawn or over a garden bed.
Many people are keen to reduce their use of water, so they don’t use a hose to clean off the soap but a bucket of clean water.