One way you could approach this problem is to use math. By math, I am referring to the theory of constraints (TOC). For example, a physical constraint is something like the physical capacity of a machine and a non-physical constraint might be the demand for carwash services. TOC is normally used to solve bottleneck problems but it can also be used to shift away from conventional accounting methods based on standard or activity-based costing.
Since you have described your 3-bay as being ?slow,? I would expect your goal is to make money now and in the future. If so, you need to make a bridge between net profit and return on investment.
To judge if your decision (e.g. adding an in-bay, tearing down the wash to build something else like a mini-tunnel, maintain the status quo, etc.) will move your business towards its goal, you need to answer several questions; how much money will be generated (throughput); how much money will be captured (investment); and how much money will have to be spent to operate (operating expense)?
These three measures are sufficient to make the bridge between net profit and ROI and the various investment decisions you want to consider. These formulas show this bridge.
Net profit = total throughput ? operating expense
Where, total throughout is the contribution margin (selling price minus cost of materials)
ROI = net profit/total investment
Productivity = throughput/investment
With these measures, you can make decisions by examining the effects of those decisions on your business? overall throughput, investment and operating expense.
A decision that results in increasing throughput, decreasing investment, decreasing operating expense or increasing productivity will generally be a good decision.
These are excerpts from several articles that I wrote on this subject that were published in the carwash trade journals. I have used this approach to help clients make well-informed investment decisions.
Hope it helps.
