Alternatively, you can use a white alpha channel in the actual texture file, then adjust the alpha using the tire's XML;
DiffuseMultiplier="(R; G; B; A)"
R = red
G = green
B = blue
A = alpha
Values can range from a minimum of-255 to a maximum of 255. So basically you would DECREASE the alpha value (if you start with a white alpha, of course) until you get the mud opacity you want.
@mexican_420 fun fact i tend to have test tires set to (7;7;7;50) and just add a nice specular map to them to add some rubber like reflections, i use to use them in mypersonal mods just because i never found a nice rubber ish like color until i meet substance, now i just have a presset wich i edit for each set of tires, including their alpha layers
@Riskyisky make sure you only have one uv set for the tires, the game reacts in weird ways to have many on a single mesh