I run a very similar operation 110' tunnel and 9 bay self serve. We have 20 vacuums on site and they bring in a good amount of money. Based off rough calculations we'd need to bring in an average of 22 extra customers per day just to break even. I guess it really varies from site to site but I'm having a tough time wrapping my head around it.
PT- I'm gonna try to break it down for you. I don't know your exact setup, prices, revenue etc so just take this from someone who's done it before and it worked
First: Your self serve will get a nice bump. Maybe this can justify a small price increase in your self serve starting cost. $2.50 to $3.00 is a nice initial 20% increase in price. If you're at $3 already, maybe you can lower your starting time from 4 mins to 3:30(or something similar). I did not need to do either of these as I was at $3 for 4 mins but would not have been opposed to doing something like this to help justify making my vacs free.
Second: Do you sell anything else at your location? Detailing? Unlimited wash club for your tunnel? gas? c-store? Cigar shop(yes, I have one of these on premises)? All of these ancillary businesses will also benefit greatly from more foot traffic at your vacs.
Third: If you decide to try this, you need to be committed to it for at least 3 months for many reasons. the big reasons being that free vacs spread word of mouth. It takes a while for this to happen. Also, the average customer only washes a few times per year so you need to give them a chance to know a bout your new free product.
Upside math examples:
Lets say your vacs were bringing in $3000 a month which is $100 day at a $1.00 start price.
Lets say your tunnel per ticket average is $10 per wash.
at $100 a day, you only need 11 extra cars a day to wash thru your tunnel to basically break even. Do you think free vacuums will be enough of a draw that 11 extra customers will wash a day? seems like a no brainer to me. If you are lucky enough to drive this free vac business to a greater revenue source like a detail or monthly unlimited club, its a homerun.
my monthly club averages $25. My average monthly customer stays on for 9 months. If I sign up only 1extra person a day to this club, I've increased revenues $225( $25 avr X 9 months). My monthly club grew from over 1500 members since I made the vacs free. Coincidence? maybe, maybe not.
Downside Math: Lets use 3 months as a test to see if this works. At the $3000/month in vac revenues example above, you'd be out $9000 if you didnt increase revenues at all. Thats not too shabby if you had to chalk it up to an "advertising promotion." This is the worst case scenario where your car count doesnt even increase by 1 car, or 1 extra flex sale or 1 extra self serve cycle etc. Its basically impossible to "lose" the entire $9k. If you "only" see an uptick in 5 extra cars a day, well then your 3 month experiment only cost you $4500 which is really cheap for advertising.
For simplicity sake, I'm leaving out all other small costs(repairs, cleanouts, utility bills etc)
I started this free vac experiment at my wash during the last summer on the east coast and ran it thru dec 31st. I purposely did not start it in the spring when I knew my vacs get heavily used after the winter season. I took my slowest 3 months to mitigate the "damage" or loss if this didn't work.
Lastly, free vacs will make your paying customers loyal to your business. They won't vac for free then wash somewhere else(unless your wash is bad). That's also a huge benefit which is tough to quantify.
hope this helps.
PM me anytime and I'll shoot you my phone number if you need to discuss further