Earl,
I didn't see that you asked for a cut off limit in car counts? either way my one XPT handles over 250 cars per day in one lane. Most high volume sites have 2-3 lanes (Jimmy J's Car Bucks has 3) and do 600-900 CPD or 200-300 cars per machine. In transaction time they probably peek out (depending how simple you program them and cash vs CC sales) at 70 -80 CPH per machine.
My highest day was about 250
coins dispensed. It doesn't matter if it's a high volume site, only how many cars one machine can handle and the pricing points that generate a combination of change.
I have one big advantage in my layout. I have a casheir room 40 feet from the XPT so I can break larger bills and not dispense all of my
coin. The biggest frustration comes from customers who insist on paying with a $20 for a $5 wash. Usually they turn in 50-70
coins a day for paper money. The main goals in adding the XPT were:
Less cash handling by employees
Faster transaction time
More accurate queuing
Less thieft of service
Decrease in payroll
Auto cashiers are applicable in any size and any type wash... not just high volume.