Cap rate (net operating Income divided by asking price) is the best way to compare apples to apples when you're looking at a cash flow stream for any investment: rent houses, car washes, apartments, office building...
The gross income multiplier doesn't take expenses into account, so there's no way to accurately gauge the true profit potential of the car wash.
If I'm looking at two different car washes and trying to find the one with the best profit potential, I'll ask the owners for their actual income and expense numbers (if they don't have it, I keep walking)
Wash #1 - owner is asking $800,000 for his wash with $100,000 NOI - or a 12.5% cap rate
Wash #2 - owner is asking $1,200,000 for his wash with $100,000 NOI - or a 8.3% cap rate
Assuming the two locations are roughly equal in location quality, age of components, etc, then I'd take the higher 12.5% return on my money.
If wash #1 needs a ton of work, and wash #2 has been newly renovated, then I'd base my decision on how much money I have to spend on the front end of the project, and how much work I'm willing to do.
With respect to cap rates, "safer investments" like apartments and office buildings in my area are usually priced at a 4 to 6% cap rate. That assumes that the buyers of those properties aren't interested in getting their hands dirty or working onsite, so they'll accept a lower return on their investment. Car washes often get priced at 10-12% cap rates because the industry is known for hands-on owners who want to get paid a good return for all of the crap that they have to deal with in running a car wash.
Disclaimer: this is just my .02 from many years buying/selling commercial real estate. I'm a newbie in the car wash realm, but using the cap rate model, I feel like I got exactly what I paid for when I bought the wash I currently own.