Braking and Grip
100 to 0 and 60 to 0: Comparing Braking Tests
A 60 to 0 mph figure is not 100 to 0 km/h scaled down. The speeds differ by 3.44 km/h and distance scales with the square, so converting between them needs the equation, not a ratio.
Two braking benchmarks are in circulation and they are not interchangeable. 100 to 0 km/h is the European figure, 60 to 0 mph the American one, and 60 mph is 96.56 km/h. Because distance scales with the square of speed, the American benchmark starts from 6.8 per cent less energy and produces a figure about 6.8 per cent shorter on the same car. At 1.0 g that is 39.3 m against 36.7 m.
Converting between them properly
Stopping distance is d = v² / (2a), so the ratio between two benchmarks at the same deceleration is the ratio of the squares of their speeds. For 96.56 km/h against 100 km/h that is 0.9324, meaning a 60 to 0 mph figure is 93.24 per cent of the 100 to 0 km/h figure for the same car on the same day.
| Deceleration | In g | 100 to 0 km/h | 60 to 0 mph | Difference |
|---|---|---|---|---|
| 5.8 m/s² | 0.59 g | 66.5 m | 62.0 m | 4.5 m |
| 9.81 m/s² | 1.00 g | 39.3 m | 36.7 m | 2.7 m |
| 11.77 m/s² | 1.20 g | 32.8 m | 30.6 m | 2.2 m |
| 13.73 m/s² | 1.40 g | 28.1 m | 26.2 m | 1.9 m |
Two to three metres does not sound like much, and in a table of performance cars separated by one or two metres it decides the order. Comparing a European figure against an American one without converting ranks the test convention rather than the car.
What else has to match
The speed benchmark is the easiest difference to correct and the smallest. Four others are larger and none is usually stated.
- Surface. Type approval braking tests specify a high adhesion surface. A public road is not one, and a prepared proving ground surface flatters every figure measured on it.
- Tyre. Braking is a tyre test before it is a brake test, and friction runs from 0.7 to 1.0 for street tyres against 1.5 to 2.0 for slicks. A track compound changes the result far more than any brake specification.
- Temperature. A summer compound below about 10 °C surface temperature loses 15 to 25 per cent of its grip, which lengthens a stop by the same proportion.
- Which stop. A first stop from cold brakes and a fifth stop from hot ones are different measurements. Published figures are usually a best of several, which means warm tyres and warm brakes.
Why the regulation measures a mean, not a peak
UN Regulation 13-H specifies a mean fully developed deceleration rather than a peak value, and requires at least 5.8 m/s² for category M1 vehicles in a Type-0 test from 100 km/h with the engine disconnected on a high adhesion surface. That corresponds to roughly 67 m.
The reason for measuring a mean is that deceleration is not constant through a stop. It builds as pressure rises and pads bite, holds through the middle, and changes again at low speed as load transfer and anti-lock behaviour shift. A peak figure would describe an instant rather than a stop, and it is the mean that determines the distance.
The regulatory minimum also puts marketing claims in perspective. A performance car braking at 1.2 g is achieving 11.77 m/s², which is more than double the legal requirement.
A short procedure
Convert every figure to one benchmark before comparing. Multiply a 100 to 0 km/h distance by 0.9324 to get the equivalent 60 to 0 mph figure, or divide by the same number to go the other way. Then check whether the tyres were the same, and treat any remaining difference under about a metre as noise, because that is inside the run to run scatter of a good test.
Questions readers ask
Can I compare a 60 to 0 mph figure with a 100 to 0 km/h one?
Only after converting. 60 mph is 96.56 km/h and stopping distance scales with the square of speed, so the American figure is 93.24 per cent of the European one for the same car. At 1.0 g that is 36.7 m against 39.3 m.
How do I convert between the two?
Multiply a 100 to 0 km/h distance by 0.9324 to get the 60 to 0 mph equivalent, or divide by it to go the other way. The factor is the ratio of the squares of the two speeds, since d = v² / (2a).
What is mean fully developed deceleration?
The average deceleration through the effective part of a stop, rather than the peak value. UN Regulation 13-H uses it because deceleration is not constant: it builds as pressure rises, holds, then changes at low speed. The mean is what determines the distance.
What does the law actually require?
At least 5.8 m/s² for category M1 vehicles in a Type-0 test from 100 km/h with the engine disconnected on a high adhesion surface, which is roughly 67 m. A performance car braking at 1.2 g achieves 11.77 m/s², more than double the requirement.
What matters more than the benchmark difference?
The tyre and the surface. Friction runs from 0.7 to 1.0 for street tyres against 1.5 to 2.0 for slicks, and a cold surface costs 15 to 25 per cent of grip. Both effects dwarf the 6.8 per cent difference between the two speed benchmarks.
Sources
- UN Regulation No. 13-H, on the Type-0 test, mean fully developed deceleration and the high adhesion surface requirement.
- Autosport technical forum, tyre coefficient of friction, on friction values for street and racing tyres.
- Regulation (EU) 2020/740, Annex I, on the wet grip classification, itself defined by a braking test from 80 km/h.
Distances are calculated as d = v² / (2a) from the moment full deceleration is established, using 1 mph = 1.609344 km/h.