Days Supply, Turn Rate, and Aged Inventory: The Math That Kills Dealership Profit

Every dealership is, at its core, an inventory business. The store buys cars at one price, finances them on a floor plan line, holds them for some number of days, and sells them at another price. The spread between those two prices — minus the carrying cost of every day in between — is the front-end gross. That single sentence is why inventory metrics are the most important operating numbers in a dealership.

Yet inventory is also the metric most operators get wrong. Sales managers want more units on the ground because more units feel like more opportunity. Owners want fewer units because every car is a line item on a floor plan bill. The job of a disciplined operator is to size inventory to actual demand — not to feel, not to brand pressure, and not to last quarter’s sales pace.

Two numbers govern that decision: days supply and turn rate. They are mathematically the same idea looked at from opposite ends of the telescope, and together they tell you whether a store is positioned to make money or positioned to bleed it.

The Two Numbers That Govern Inventory

Days supply is the simpler of the two. The formula is (current inventory units divided by monthly sales rate) multiplied by 30. A store sitting on 180 new units that sells 60 per month is carrying 90 days’ supply. The number answers a single, blunt question: at today’s pace, how long would it take to sell through what is sitting on the lot if no new units arrived?

Turn rate is the inverse view. The formula is annual sales units divided by average inventory. The same store — 60 units a month at 180 units of average inventory — sells 720 a year and turns its inventory four times. New-vehicle operators tend to think in days supply because that is how the manufacturer talks. Used-vehicle operators tend to think in turns because that is how the wholesale market talks. Both are describing the same physics.

What matters is that an operator looks at both. Days supply tells you how exposed you are to floor plan interest and market depreciation right now. Turn rate tells you whether the operating engine is producing the velocity it should be producing across a full year. A store can have an acceptable days supply on a given Friday and still be running an unhealthy turn rate over twelve months — and that is the case where inventory discipline quietly erodes profit.

What Counts as “Healthy” Days Supply

Industry sweet spots are well established. New-vehicle inventory tends to run healthiest in the 60 to 75-day range. Used-vehicle inventory tends to run healthiest in the 30 to 45-day range — used depreciates faster, so the cost of being wrong is higher and the discipline has to be tighter. Translated to turns, that is roughly 5 to 6 turns per year on new and 8 to 12 turns per year on used.

Those are starting points, not commandments. The “right” days supply varies by brand, by market, and by the turn velocity of specific models. A luxury franchise can run hotter — 80 to 100 days is often acceptable because individual units carry higher gross and customers expect a deeper selection to configure against. A high-volume mass-market store needs to run leaner because gross per unit is thinner and the math only works if velocity is high.

Within a single store, the same logic applies model by model. A pickup configuration that sells in 21 days deserves to be re-stocked aggressively. A trim level that consistently ages past 75 days deserves to be ordered less, priced more sharply at receipt, or removed from the order guide entirely. Days supply at the store level is an average — the discipline lives one VIN at a time.

The Aged Inventory Death Spiral

Every market has units that won’t sell at intended margin. The wrong color, the wrong trim, the wrong powertrain for the season, a competitor incentive that just landed — there are a hundred reasons a unit can stop moving. The discipline is not pretending those units don’t exist. The discipline is recognizing them early and acting before the math gets worse.

The industry rule of thumb is that 60-day-old units start losing margin power. Pricing tools recommend the first markdown, sales managers stop showing the unit first, and the customer pool that would have paid sticker is mostly gone. By 90 days, the unit typically needs to be wholesaled at a loss or held through a second markdown that erases what front-end gross was left. Every additional day on the ground is roughly $40 to $60 in floor plan interest, plus an unmeasured but real amount of market depreciation, plus the opportunity cost of the lot space and the capital.

The reason aged inventory compounds badly is that the longer a unit sits, the more the surrounding economics work against it. Floor plan interest accrues daily. Market values drift as new model years arrive. Reconditioning expenses creep up as detail and mechanical issues recur. Salespeople lose interest in working a unit they have failed to sell three times. Aged inventory is not just a one-time write-down — it is a slow drain that quietly subtracts from store profit every single month it isn’t addressed.

Used Vehicles: A Different Discipline

Used inventory is where the most disciplined operators separate themselves. The new-vehicle world has factory pricing, factory incentives, and a finite order guide. The used-vehicle world has none of that — every unit is sourced individually, priced individually, and depreciates on its own clock. Get used wrong and the losses scale much faster than on new.

Modern used-vehicle operations run on price-to-market tools — vAuto, ProfitTime, MarketScan, and similar platforms — that rank every unit against live competing inventory inside the store’s market radius and recommend a markdown cadence based on age and price position. A unit priced 96% to market on day 15 is in a very different position than the same unit priced 102% to market on day 45, and the system flags the difference long before a human manager would catch it.

The discipline isn’t the tool — it’s using the tool. Disciplined used-vehicle operations review the price-to-market and age report every Monday morning, mark down units on a fixed cadence, and accept that a 30-day-old unit priced sharply is worth far more to the business than a 75-day-old unit being held for an extra $500 of theoretical gross. That single behavior — pricing today’s reality instead of yesterday’s hope — is what separates a 10-turn used operation from a 6-turn one.

Wholesale Exit — When to Cut Bait

The hardest decision in inventory management is taking a known loss today to avoid a larger unknown loss tomorrow. A 75-day-old unit that won’t respond to the second markdown is a wholesale candidate. Holding it for another 30 days while hoping for a retail customer typically costs another $1,500 in floor plan, market depreciation, and reconditioning — to recover, at best, a few hundred dollars more than the auction would have paid. The math almost always favors the exit.

Disciplined operators set policy rather than negotiate every unit. The rule at a well-run store is something like: any used unit at 60 days gets a final markdown; any used unit at 75 days that hasn’t responded goes to the wholesale list; any used unit at 90 days is gone, period. The policy removes ego from the decision. It also removes the temptation to keep one ugly unit on the lot because someone in the store is convinced it will sell next week.

Healthy stores wholesale a steady, modest percentage of their used acquisitions. Unhealthy stores either wholesale almost nothing — because they hold units indefinitely and let aged inventory metastasize — or wholesale far too much, because their acquisition discipline at the front end was poor. The wholesale exit rate is, in practice, one of the cleanest indicators of how well the inventory engine is being run.

How Operators Win on Inventory Discipline

Every dealership we acquire arrives with some version of an inventory problem. Sometimes it’s overstock — too many units, too much days supply, too much floor plan interest. Sometimes it’s aged — units that quietly crossed 90 days under the previous operator and are now sitting on the books at numbers that no longer reflect the market. Sometimes it’s mix — the right total unit count but the wrong models, trims, and colors for the local customer base.

The fix is rarely a single dramatic action. It is a set of weekly disciplines: age report every Monday, price-to-market review every Wednesday, wholesale list finalized every Friday, order guide adjusted at every monthly cycle based on what actually sold versus what was ordered. None of it is glamorous. All of it compounds. Within two to three months of consistent execution, days supply normalizes, aged percentages drop, turn rate rises, and floor plan interest as a percentage of gross falls measurably.

That sequence — diagnose the inventory inheritance, install the weekly disciplines, hold managers accountable to the metrics, accept the short-term wholesale losses to clean the deck — is what produces the margin expansion that shows up six and twelve months after a Coleman Prime acquisition. It is not magic. It is the inventory math, applied with the same discipline at every store, every week, with no exceptions.