Munich – September 10, 2026 -- German auto club ADAC has found that winter tires start losing significant safety performance once tread depth drops below four millimeters, well above the legal minimum of 1.6 millimeters allowed in Germany.
ADAC tested five tires worn down to specific depths
Researchers took five winter tires from different manufacturers and price ranges and used a specialized machine to wear them down to four millimeters and two millimeters of remaining tread. The tires then underwent handling and braking tests on wet and snow-covered surfaces.
Aquaplaning risk jumps sharply as tread wears down
Using ADAC's standard tire-test safety ratings as a baseline, the study found that longitudinal aquaplaning performance dropped by an average of 12 percent between new tires and those with four millimeters of tread. At two millimeters, that loss doubled to 22 percent. Lateral aquaplaning -- the kind drivers encounter in curves -- showed an even steeper drop of 25 percent on average.
Braking on snow takes far longer with worn tires
Snow-braking performance fell by 20 percent between new tires and those worn to two millimeters. In one example cited by ADAC, a Bridgestone tire braking from 30 km/h on snow would have shed just over half its speed by the time an identical new tire had already come to a complete stop.
Performance loss accelerates suddenly near the legal limit
The pattern held across nearly all test categories, according to ADAC. In several cases, the drop in performance between a new tire and one at four millimeters was similar in magnitude to the drop between four and two millimeters -- meaning tire performance can fall off a cliff just before reaching the legal minimum tread depth.
ADAC recommends replacement at four millimeters, not 1.6
Based on the findings, ADAC is advising drivers to replace winter tires once tread depth reaches four millimeters rather than waiting for the legal 1.6-millimeter threshold. The club noted that even premium-tier tires cannot compensate for insufficient tread once wear sets in.