There are always factors of uncertainty in tyre testing which may, at worst, lead to incorrect conclusions. Acknowledging and controlling these factors is an essential part of tyre studies.One factor of uncertainty in investigations of studded tyres is stud protrusion. If changes in stud protrusion are not controlled in the course of the test, this will inevitably lead to incorrect evaluation of the differences between different tyres. This is most probably the main reason for the observed contradiction between different test results. In order to control changes in stud protrusions, a so-called two-protrusion method was developed. For each tyre model to be studied in the test, two sets of test tyres are used with different stud protrusions. Both sets of tyres are examined, and the final results are given with linear corrections for all tested tyres to correspond to a certain stud protrusion, e.g. 1.2 mm.When the so-called reference tyre, used to control the changes in test conditions, is a studded tyre, with protrusions changing during the course of the test, the risk of drawing incorrect conclusions is great. The stud protrusion of the reference tyre can be standardised by using glued studs. The experience from this method is encouraging. The method works especially well when the stud protrusion used in the reference tyre is as close as possible to that of the studied tyres.The grip of a winter tyre depends largely on weather and road conditions. A studded winter tyre is at its best in slippery conditions on smooth ice, whereas on rough ice and on snow an unstudded winter tyre can reach a good friction level, comparable to that of some studded tyres.Wear may have a notable effect on the grip of the tyre. The grip of studded tyres will usually slacken with wear more than that of friction tyres.