Accidents related to insufficient road grip in Sweden have historically been investigated for winter conditions, where substantial effect on passenger car accident risk from different types of tyres has been observed. Grip-related accidents during summer conditions have so far primarily focused on the effect of Electronic Stability Control (ESC), with no assessment of possible influence from tyre properties. The use of winter tyres during summer has been increasing among passenger cars in Sweden and is currently estimated to be around 7%. Tyre tests indicate 20%–40% longer braking distances for winter tyres compared to summer tyres, and statistics have indicated that passenger cars with winter tyres are heavily overrepresented among the fatal accidents during the summer period in Sweden. The purpose of this study has therefore been to estimate the potential risk increase for a fatal grip-related accident for passenger cars using winter tyres, compared to summer tyres, during summer. Statistics from in-depth studies of fatal road accidents in Sweden from the 10-year period 2012–2021 was used in an induced exposure risk analysis, with a complementary stratified analysis investigating the effect of possible confounding factors. The results indicate that the overrepresentation among fatal accidents of cars equipped with winter tyres during the summer period to a large extent is related to the inferior road grip of winter tyres. The risk increase was particularly high for fatal grip related accidents when driving within the regulated speed limit. Tyre properties have generally been neglected as a contributing factor among fatal accidents occurring in the summer period in the Swedish in-depth studies. The present study shows that tyres have a substantial impact on grip-related fatal accidents with passenger cars in Sweden during the summer period, which account for 25% of the fatal accidents involving passenger cars. This highlights the need for increased attention to tyre properties in accident investigations. The results are most likely relevant also to other countries where winter tyres are used and the findings should be addressed by policy makers and other stakeholders.