Fuel comparison

Petrol vs diesel, at the MOT bay.

Petrol cars pass first time at 77.2%. Diesels run 77.0%. Across 61,146,299 tests last year, that's a 0.2-point gap and the two segments draw.

Petrol

77.2%

first-time pass rate

Tests

32,592,230

Avg mileage

69k

Models

1,325

Diesel

77.0%

first-time pass rate

Tests

28,554,069

Avg mileage

90k

Models

830

The headline gap reads larger than it is. UK diesels at the MOT bay are an older fleet — average mileage at test is 90k versus 69k for petrol. Newer petrol stock skews the petrol number up; diesel particulate filters and EGR valves on long-mileage cars skew the diesel number down. Year-on-year the gap is closing, not widening.

Same model, different fuel — who wins?

Models that sell in meaningful volume on both fuels (5,000+ UK tests on each variant). Pass rate is for the model overall — fuel-specific splits coming when the year-band ETL lands.

What each fuel fails for.

Top defect codes for cars where the fuel listed is the dominant variant. Same fleet-wide checks on both, so divergence here points to fuel-specific wear.

Petrol — top defects

01
A rear registration plate lamp or light source missing or inoperative in the case of multiple lamps or light sources
526,846
02
A suspension pin, bush or joint excessively worn
370,356
03
Tyre tread depth not in accordance with the requirements
364,326
04
A spring or spring component fractured or seriously weakened
290,256
05
Headlamp reflector or lens slightly defective
215,650

Diesel — top defects

01
Windscreen or window damaged or seriously discoloured but not adversely affecting driver's view
687,711
02
A rear registration plate lamp or light source missing or inoperative in the case of multiple lamps or light sources
605,834
03
Tyre tread depth not in accordance with the requirements
301,311
04
A transmission shaft constant velocity joint boot severely deteriorated
212,795
05
A suspension pin, bush or joint excessively worn
204,025

Where next

Compare any two UK cars side-by-side, or browse the full fuel-segment leaderboards.

Compare two cars → All petrol cars All diesel cars