Work it backwards, not forwards
The usual way to get chemical cost per car is forwards: take the dilution off the label, multiply by the volume the applicator throws, price the concentrate. Every step in that chain is an assumption, and the dispenser is the weakest one. Working backwards skips all of it. A 55 gallon drum of presoak at 480 dollars that lasted 2,900 cars cost 16.55 cents a car, and that figure already contains whatever the equipment was actually doing.
Four products at the defaults come to 35.46 cents a car, or 35.46 dollars per hundred cars. On a 14 dollar average ticket that is 2.53 percent.
The gap is the point
The presoak chart says 1.9 fluid ounces a car. The drum says 2.43. That is 27.8 percent more product than the setting calls for and 3.6 cents a car, which over 60,000 cars is 2,158 dollars. Nothing on the invoice shows this, nothing on the dispenser shows it, and the only place it is visible is in the division above.
Drift runs both ways and the pleasant direction is worth more suspicion than the unpleasant one. A drum that outlasts its setting usually means a short draw, and a short draw shows up in the wash quality long before anybody reads it in a ledger.
Per hundred cars, because per car rounds to nothing
Tire cleaner at the defaults costs 3.31 cents a car. Rounded to the nearest cent that is three cents; rounded the way money usually gets rounded in a spreadsheet it can vanish entirely. Per hundred cars it is 3.31 dollars, and per year it is 1,985 dollars. The unit is chosen so the number stays visible.
What a drum cannot tell you
The division cannot distinguish product that went through an applicator from product that left the drum some other way. A leaking foot valve, a flushed line, a jug filled for the wheel bay and a drum that was already part used when it was cracked all read as draw. When a figure looks wrong the arithmetic is usually right and the story about the drum is usually wrong, which is a good reason to write the car count on the drum in marker on the day it goes on.
Questions people ask
How much does chemistry cost per car at a car wash?
Whatever your own drums say it does, which is the only reliable way to get it. Divide what a drum cost by the number of cars that went through between putting it on and running it out. At the defaults on this page four products come to 35.46 cents a car, but that figure is entirely a function of the products, the equipment and the settings on one particular site.
Why work chemical cost backwards from the drum?
Because working it forwards from a label needs a dilution ratio and an applicator rate, and the dispenser is the part of that chain that drifts. Drum price divided by cars per drum contains whatever the equipment was really doing, with no assumption in it at all. That is also why it can be compared against the setting to find drift.
What does it mean if a drum lasts longer than expected?
Usually that the dispenser is drawing short of its setting, which will show up in wash quality before it shows up anywhere financial. It can also mean the interval you measured spanned a quiet period, or that the car count is off. The page prints the drift in both directions rather than treating a long-lasting drum as good news.
Why report chemical cost per hundred cars?
Because per car it is fractions of a cent for several products and rounds to nothing. Per hundred cars the same numbers are visible money, and over a year they are the difference between two sites with identical revenue. The page shows all three so the small figure never disappears into a rounding.
Is the cheapest concentrate the cheapest to use?
Not reliably. Price per ounce and draw rate move independently, so a dearer drum that draws less can beat a cheaper one that draws more. The page prints both columns for exactly that reason, and it is also why a supplier switch has to be re-measured over a real drum rather than assumed from the new price list.