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Unless you’ve been hitting the gym hard enough to bench-press a sedan, you’ll definitely want to drive.
While 50 meters is a very short distance—roughly the length of an Olympic swimming pool—it’s a bit difficult to wash a car that isn't actually at the car wash.
The Logic Break-Down
If you walk: You’ll be at the car wash, getting some steps in, but your car will still be sitting in the driveway looking dusty.
If you drive: Both you and the vehicle arrive at the destination, which is generally a prerequisite for the "washing" part of the plan.
One exception: If you are just walking over to buy cleaning supplies or to see if there's a long line before pulling the car out, then walking is the way to go!
Would you like me to check the local weather for you to make sure it's not going to rain right after you finish?