MOT performance
Mercedes-Benz SLK vs BMW Z4: MOT pass rates compared
In 2023–2024, 74.2% of 133,811 Mercedes-Benz SLK MOT tests and 76.9% of 70,690 BMW Z4 tests passed first time — 2.7 points in favour of the BMW Z4. Once age, mileage and fuel are allowed for, the BMW Z4 fails less often than the Mercedes-Benz SLK: 0.68× against 0.82× the national rate for comparable cars.
133,811 Mercedes-Benz SLK and 70,690 BMW Z4 MOT tests, 2023–2024
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| Figure | Value | Basis |
|---|---|---|
| Mercedes-Benz SLK: first-time pass rate | 74.2% | ±0.2 pts (95%), n = 133,811 |
| BMW Z4: first-time pass rate | 76.9% | ±0.3 pts (95%), n = 70,690 |
| Mercedes-Benz SLK: failures vs similar cars | -18% | same age, mileage and fuel = 1.00 |
| BMW Z4: failures vs similar cars | -32% | same age, mileage and fuel = 1.00 |
| Mercedes-Benz SLK: dangerous defect | 6.7% | n = 133,811 |
| BMW Z4: dangerous defect | 7.2% | n = 70,690 |
| Average age at test | 13.5 vs 13 years | Mercedes-Benz SLK vs BMW Z4 |
Why the raw pass rates can mislead
The Mercedes-Benz SLK was tested at an older average age (13.5 years against 13), and older cars fail more often whatever they are. That is why the table above also shows failures against cars of the same age, mileage and fuel: it compares the models, not their fleets.
Failure areas side by side
Share of MOT tests that failed on each area, and in brackets how that compares with cars of the same age, mileage and fuel (1.00 = national average). The clearest difference is road wheels, where the BMW Z4 fails 3.0 times as often on an adjusted basis.
| Area | Mercedes-Benz SLK | BMW Z4 |
|---|---|---|
| Brakes | 5.6% (-29%) | 5.5% (-34%) |
| Steering | 0.8% (-61%) | 0.3% (-85%) |
| Suspension | 10.3% (Same) | 6.5% (-44%) |
| Tyres | 6.3% (+11%) | 5.9% (-3%) |
| Road wheels | 0.4% (+40%) | 1.5% (+320%) |
| Lights, reflectors and electrical | 7.6% (-42%) | 6.3% (-58%) |
| Body, chassis and structure | 1.3% (-72%) | 0.9% (-81%) |
| Driver's view of the road | 2.9% (-46%) | 4.0% (-31%) |
| Noise, emissions and leaks | 2.4% (-43%) | 3.1% (-36%) |
| Seat belts and airbags | 1.1% (-16%) | 1.5% (+10%) |
| Registration plates and VIN | 0.4% (-13%) | 0.4% (-7%) |
| Speedometer and driving controls | 0.0% (-40%) | 0.0% (-73%) |
Pass rate at the same age
Comparing cars of the same age removes most of the fleet-age effect from the raw rates.
| Age at test | Mercedes-Benz SLK | BMW Z4 |
|---|---|---|
| 3-4 years | 91.4% (n = 3,424) | 90.2% (n = 5,681) |
| 5-6 years | 86.5% (n = 9,869) | 85.2% (n = 1,645) |
| 7-8 years | 82.7% (n = 16,294) | 85.0% (n = 5,360) |
| 9-10 years | 77.9% (n = 19,611) | 81.8% (n = 7,083) |
| 11-12 years | 75.1% (n = 15,639) | 81.4% (n = 7,641) |
| 13-15 years | 69.6% (n = 15,533) | 76.6% (n = 13,253) |
| 16+ years | 67.6% (n = 52,803) | 68.8% (n = 28,075) |
Most common failure item
- Mercedes-Benz SLK · coil spring: fractured or broken (4.51% of tests)
- BMW Z4 · condition: has ply or cords exposed (2.09% of tests)
MOT results reflect how cars are maintained as well as how they are built, so a gap between two models is a difference in test performance, not proof that one is more reliable. For one car, its own MOT history says more than any average.
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About this data. These are MOT test results, not a measure of mechanical reliability: a car's MOT outcome reflects its condition on the day, which depends on how it has been maintained and driven as well as how it was built. Figures cover class 4 vehicles (cars and light vans), normal tests only (retests excluded), outcomes pass, fail and pass after rectification at the station; vehicles at least 2.5 years old at test. Tests from 2023-01-01 to 2024-12-30. “Compared with similar cars” means compared with the national rate for cars of the same age, mileage band and fuel. Contains public sector information licensed under the Open Government Licence v3.0. Makes and models are matched to 73 Auto's vehicle catalogue; tests that can't be matched are left out rather than guessed. See how the MOT figures are produced.