Road wetness can lead to a significant loss in tyre traction. Although a driver can easily distinguish between dry and wet roads, the thickness of a water film on the road (wetness) and its impact on the vehicle dynamics are more difficult for a driver to classify. Furthermore, autonomous vehicles also need a graded classification of road conditions. There are known sensors, which are able to classify road conditions, but these are either not able to quantify the road wetness or are not suitable for mass production. Therefore, this work analyses a method to measure the road wetness by analysing tyre spray with plain acceleration sensors at positions like wheel arch liner or side skirt. It discusses influences of vehicle speed, road wetness, tyres, road structure and sensor positioning. The results show that a quantification of road wetness is possible, but it relies on the sum of all boundary conditions.