If you see M+S, M/S, M&S, or just MS on a sidewall, that tire meets the Rubber Manufacturers Association (RMA) baseline for mud and snow designation. It does not mean the tire will hook up well in a blizzard. The mark is purely geometric. It has nothing to do with actual performance data.
The Math Behind the Tread Pattern
To earn those letters, a tire must follow strict physical rules. The RMA requires multiple pockets or slots in the tread. These grooves must extend toward the center. They need to reach at least 1/2 inch (about one centimeter) from the footprint edge. This measurement is taken perpendicularly to the tread centerline.
The slots also have a width requirement. The minimum cross-sectional width must be 1/16 inch. The edges of these pockets must sit at a specific angle. That angle has to be between 35 and 90 degrees from the direction of travel. It’s a rigid set of blueprints.
There is also a void area requirement. The tread contact surface void area must be at least 25 percent. This calculation is based on mold dimensions. Basically, you need rows of fairly large grooves starting at the edge and moving inward. At least a quarter of the tire’s surface must be empty space. More voids mean better ability to grab through snow or mud.
Why the Mark Is Misleading
You might think that seeing the RMA mud and snow rating means you are good to go in winter conditions. It does not. The rating is based entirely on geometry. It ignores how the tire performs when actually driving.
The RMA knows this gap exists. They created a separate designation to address it. It is called “Severe Snow Use.” You can spot this rating by a picture of a mountain with a snowflake inside. It sits next to the MS designation.
Earning that three-peaked mountain symbol is harder. The tire must exceed certain traction results. Those results come from the American Society for Testing and Materials severe snow test procedures. That is actual testing. The M+S mark is just a shape check.
