This has probably happened to you. You’re reading through an auction listing for what might be your next dream ride. The car looks great in pictures, the equipment matches the spec you’re looking for, and there are stacks of receipts for professional work. Then you get the mileage and it reads “TMU.” True Mileage Unknown. Your gut sinks.

Between the growth of the collector car hobby and the rise of online auctions, the term TMU has become a common phrase. But does this designation automatically serve as a condemnation of an otherwise desirable vehicle? Not necessarily.

To understand why this term is so often used, you have to consider who uses it. After all, it’s not typically private sellers that throw the term around, but rather auctions (online, live or otherwise), consigners, and specialty dealers. These companies are conduits between past or current owners and future ones, and that TMU designation protects them from future legal issues should a vehicle they’ve sold or brokered later be found to have a less than honest history.

This separation from liability for rue also be seen in the ad language itself. A stroll through the staging area at a recent Mecum auction revealed another common phrase used frequently on vehicle descriptions – “Odometer reads: XX,XXX miles.” In effect, the auction house is saying they can’t ensure that the vehicle has only covered the mileage shown on the odometer, or that the odometer is actually the original one to the car. The responsibility lies with the buyer to accept that the mileage aligns with the condition of the car, or to at least value the purchase accordingly.

Why actual mileage matters

While unknown mileage certainly isn’t a death sentence for every car, there are some vehicles and situations where it matters a lot. Generally speaking, the rarer the car and the lower the mileage – and therefore the higher the value – the more important it is. That’s because mileage is one of the most reliable indicators (in buyers’ minds anyway) of how much use a car has seen in its lifetime.

A car that has barely been used is more likely to be 100% original than one that has been driven extensively and maintained with replacement parts and potentially careless work. So if you’re selling an all-original, numbers-matching ’60s muscle car that supposedly hasn’t turned five-digit mileage, you’d better be able to back up that odometer reading if you’re expecting top dollar.

Why actual mileage may not matter

The truth is, mileage isn’t the only indicator of condition, and odometers often lie, even if it’s not their fault. Older vehicles, for instance, used to have five-digit odometers, and it wasn’t uncommon to see “Exceeds Mechanical Limits” listed on the title of a vehicle sold with more than 100,000 miles. In fact, the owner was often responsible to report that. After all, how could the state know if the 50,276 clicks on the odometer was the second or third trip around the dial?

As many of the most popular classics are now at least 50, 60, or 70 years old, it’s not uncommon for them to have had a speedometer fail along the way. Swapping out a dead speedo with a used one was common practice, if it even got replaced at all. In recent years many owners have upgraded to modern digital instruments, and they may not have documented the known mileage at the time of conversion.

Of course, when it comes to less rare and higher-mileage vehicles, the odometer reading is far less important than the actual condition. A 30-year-old vehicle with an unverified 190,000 miles that’s been garage-kept in a salt-free environment and seen mostly highway miles could easily be as desirable as the same vehicle with 90,000 verified miles that’s been parked outside, driven through wintry conditions and used for short trips on rough roads.

Would you rather have an amazingly well kept vehicle that’s covered a quarter-million miles or a low-miles rust bucket? No two cars are the same, and mileage alone should not be your guide. And who knows, you may be able to use the unknown mileage as a negotiating point in your favor.

Validating actual mileage

If you find yourself considering the purchase of a vehicle labeled “TMU” for whatever reason and you’d like a better sense of the actual mileage to help you decide, there are a few clues you can look for to ease your mind.

• Modern vehicle history services like CarFax are great for validating the life and times of vehicles, including the date and mileage of activities like changes of ownership, registration renewal or changes, state inspections, and maintenance and repair work. However, the company has only had access to data from all 50 states since about 1993. Early history from older vehicles will not be in the system, and keep in mind the service only collects repair and service data from registered shops. Any work that has been performed by the owner or an offline business will not be in the system.

• Previous service records are one of the best indicators of a vehicle’s history. An owner that can show service receipts for oil changes, tune ups, tire replacements, warranty work, or any other documented services will instill the most confidence in a prospective buyer. Careful reading between the lines can also be very revealing. Did the owner service at the dealership or a discount shop? Did they do full scheduled maintenance or just oil changes? These answers matter as much as the odometer.

• Buying directly from the long-time owner of a vehicle can also provide valuable insight to fill in gaps between service records. Your chances of getting thorough service records (see above) are also better in this kind of transaction. Dealers often discard old service records out of fear the new buyer may contact the former owner at some point, so they simply eliminate the evidence.

• If you’re going to buy from a dealer, it may be beneficial to shop through a specialist in the make or model you’re considering. The more specialized they are, the more important their reputation is among the owner community they serve.