Mean time between failures (MTBF): calculating it for fleets

Mean time between failures (MTBF) is a fundamental indicator in vehicle fleet maintenance management: it measures how long, on average, a vehicle remains in operation between one failure and the next.

It is an indicator that helps you understand fleet reliability and identify trends or recurring problems.

By using this metric, you can optimize the fleet, improving preventive maintenance, reducing unexpected downtime and increasing operational efficiency.

Before getting into the calculation, it is worth establishing two things responsible for most calculation errors. MTBF stands for mean time between failures—the average time between failures, not repairs. And the indicator is always time divided by the number of failures, never the reverse: if you invert the operands, the result stops being a time and becomes a frequency.

Here is a closer look:

• What mean time between failures is
• Why use this maintenance indicator
• How to calculate mean time between failures in the fleet
• What analyses you can perform using MTBF
• How to use the results to improve fleet maintenance

What is mean time between failures?

Mean time between failures, or MTBF, is an equipment or vehicle reliability indicator: it summarizes in one number how often that asset stops because of a failure. It originated in industrial maintenance but has become popular in vehicle management.

Representing the average time between failures, MTBF helps you understand how often a vehicle needs repairs. This knowledge is essential for properly planning maintenance and operations. In practice, it answers the question every maintenance manager asks when looking at a truck: how long, on average, does this vehicle run before returning to the workshop?

It is worth separating two concepts often treated as synonyms. MTBF concerns failure frequency. Vehicle availability also depends on how long each stop takes: a truck may rarely fail and still spend weeks out of service if each repair takes a long time or a part takes time to arrive. The two indicators are read together, and neither replaces the other.

Why use this maintenance indicator?

The importance of using this indicator lies in increasing fleet productivity, avoiding financial losses and gaining a deeper understanding to prepare for and prevent likely vehicle failures.

Using MTBF in your management is not mandatory, but it can make your maintenance control plan (MCP) much more effective. After all, it provides a more strategic view for making informed repair and maintenance decisions.

A caveat has accompanied the indicator since its industrial origins: MTBF is an average. It describes the behavior of the group over the period analyzed and does not predict the date of a specific vehicle's next failure. Read as a point prediction for a single vehicle, it disappoints; read as a trend signal, it is one of the most useful maintenance indicators a fleet can keep track of.

How do you calculate mean time between failures in the fleet?

1 – Gather the necessary data

To calculate this indicator, you need: total vehicle working (or operating) time, downtime for corrective maintenance and number of stops.

• Total operating time: refers to the hours the vehicle was actually in use (working). It is important to check whether corrective maintenance hours are included or have already been subtracted before applying the MTBF formula. This is where calculations most often go wrong: if you start with calendar time for the period, downtime is still included and will be deducted in the formula; if you start from a record that already excludes downtime, deducting it again removes the same time twice.
• Number of failures: count vehicle stops lasting 10 minutes or more due to vehicle problems, such as breakdowns, loose parts, etc. This 10-minute cutoff is a practical reference, not a fixed rule—it prevents trivial stops from inflating the count. The exact value matters less than keeping it constant and documented: changing the criterion changes MTBF without anything changing in the fleet.
• Total repair time: include only the time needed for repairs, covering corrective maintenance services and excluding waiting for parts or other interruptions. This does not mean waiting for a part is irrelevant—it stays outside this calculation but needs to be kept track of separately, because it is still vehicle downtime.

It is worth emphasizing that the data must correspond to the period you want to analyze, whether 24h, a week, a year, and so on. All three figures must come from the same period and use the same time unit, or the result will be meaningless.

2 – Apply the formula

To calculate mean time between failures, start with total vehicle working time (WT) and subtract total repair time (RT). Then divide the result by the number of stops in the period (S).

This is the formula:

MTBF = (WT – RT) / S

Reading it in parts: (WT – RT) is the operating time remaining after corrective downtime is deducted, and dividing it by S gives the average interval between one failure and the next. The result uses the same unit as WT and RT—if you used hours, MTBF is in hours.

An example with exact figures: a truck assessed over 30 calendar days has a WT of 720 hours; during that period, it stopped four times due to failures and used a total of 40 repair hours. The calculation is (720 – 40) / 4, or 680 / 4 = 170 hours. On average, this vehicle ran for just over seven days between failures.

The higher the MTBF result, the better. It means the vehicle is experiencing few problems and your maintenance management is effective—the indicator rises precisely when failures become rarer.

On the other hand, a low MTBF may indicate an inefficient vehicle and poorly performed repairs, for example.

Notice what the number does not say. MTBF answers “how often does it fail,” not “how long does it remain stopped.” That second question belongs to mean time to repair—MTTR—which appears within RT in the calculation above. A fleet with high MTBF and long repairs still has low availability, and looking only at MTBF would hide exactly that problem.

What analyses can you perform with MTBF?

MTBF is a metric that can contribute to different analyses in fleet management. It mainly allows:

• Greater predictability in maintenance routines

With MTBF, you can estimate more accurately when a vehicle will need preventive maintenance. This helps prevent unexpected failures, bringing appropriate services forward and reducing operating costs.

The precision of this estimate, however, depends on history: with few recorded failures, or failures concentrated in a single component, the average hides more than it reveals. It is worth keeping track of the indicator over several periods before changing the schedule because of it.

• More accurate assessment of vehicle reliability

MTBF allows you to identify which vehicles or components are more prone to failure.

This makes it easier to decide which items need special attention or earlier replacement—as well as to assess whether it is worth keeping vehicles with many faults and problems.

• Understanding vehicles' cost-benefit balance

By analyzing MTBF, you can assess the relationship between maintenance costs and vehicle useful life, optimizing the return on investment in each fleet asset.

The assessment becomes more honest when cost per kilometer driven sits alongside MTBF: a vehicle that rarely fails but has expensive parts may cost more than another with lower MTBF and inexpensive maintenance.

• Comparing different vehicles, drivers and operations

MTBF helps compare the performance of different vehicles, drivers or even routes used in specific operations. For example, you can cross-reference a vehicle's mean time between failures with the number of incidents a driver has had, recorded in the operation's checklists and work orders.

This makes it possible to understand whether the problem lies in the vehicle or in that particular driver's driving style.

Similarly, analyzing failure frequency on different routes can help identify poor, problematic roads, allowing route adjustments that reduce failures and improve MTBF.

One condition applies to all three comparisons: they only make sense between comparable situations. Comparing the MTBF of a new vehicle on a paved route with that of an old vehicle on a dirt road compares the operation rather than the asset—and leads to wrong conclusions about which truck to keep in the fleet.

How can you use the results to improve fleet maintenance?

Make adjustments based on patterns and trends

Analyze MTBF data to identify vehicles or components that fail frequently and adjust maintenance strategies to address these specific issues, minimizing future failures.

For this, besides understanding the number of failures and mean time between them, you need to identify what those failures are and how often they recur.

You can apply the MTBF formula to a specific failure: simply perform the same calculation using the number of failures and repair time specific to the failure being analyzed. Total operating time remains that of the entire period—only RT and S for that failure type change.

Optimize the preventive maintenance schedule

Use MTBF to determine the ideal frequency of preventive maintenance, prioritizing vehicles with lower MTBF for earlier attention and reduced failure risk.

The caveat about averages returns here: since some failures occur before the average value, the preventive interval is usually set below the observed MTBF, not equal to it. Scheduling preventive maintenance exactly at MTBF leaves all events occurring before the average uncovered.

This routine need not focus only on preventive maintenance services; adjusting the vehicle inspection schedule is also important.

The fleet checklist should cover the main items reported as failing most frequently, with inspection as soon as possible so they are checked before the expected time of failure.

Provide driver training

When vehicle problems caused by drivers are identified, it is important to offer training focused on proper vehicle operation and early identification of signs of problems.

This helps you make better use of all the benefits of a vehicle with high performance: longer useful life, lower costs, more efficient fuel consumption and higher MTBF, among others.

Assess and adjust the mechanic team (or outsourced service)

When MTBF is low, it may signal a problem with poor-quality maintenance service.

Review the performance of the maintenance team or outsourced service, adjusting processes and training to ensure they align with the needs identified by MTBF. Before concluding that the workshop is at fault, however, check whether the failure also recurs in vehicles serviced by different teams: when it appears across the fleet, the cause tends to lie in the component, part specification or operating conditions, rather than who performed the repair.

Want to deepen your knowledge of the mean time between failures of your fleet's vehicles and effectively apply this indicator to optimize maintenance management in your operation? Sign up now for our maintenance management course and take your career to the next level (in Portuguese).