# Regulation (EC) No 1222/2009 of the European Parliament and of the Council of 25 November 2009 on the labelling of tyres with respect to fuel efficiency and other essential parameters (Text with EEA relevance), Division 4.2.8.5: Calculation of the wet grip index of the candidate tyre

Extent: E+W+S+N.I.

The wet grip index of the candidate tyre ( G(T) ) is calculated as follows:
G(T) = μ peak,ave (T) μ peak,ave (R) × 125 + a × t − t 0 + b × μ peak,ave (R) μ peak,ave (R 0 ) − 1,0 × 10 − 2
where:
t is the measured wet surface temperature in degree Celsius when the candidate tyre ( T ) is tested
t 0 is the wet surface reference temperature condition
t 0 = 20 °C for normal tyres t 0 = 10 °C for snow tyres
μ peak,ave (R 0 ) = 0,85 is the peak braking force coefficient for the reference tyre in the reference conditions
a = – 0,4232 and b = - 8,297 for normal tyres, a = 0,7721 and b = 31,18 for snow tyres.

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Source: https://www.legislation.gov.uk/eur/2009/1222/annex/V/division/4/division/4.2/division/4.2.8/division/4.2.8.5 (the official version)
Page: https://openlegislation.uk/eu-regulation-2009-1222/annex-V/division-4/division-4.2/division-4.2.8/division-4.2.8.5
Contains public sector information licensed under the Open Government Licence v3.0. Source: legislation.gov.uk, The National Archives.