Sometimes it's not the Mirror Texture being low res but the way it's been mapped to the mirror mesh.
Does that mod have it's own mirror texture ?
What happens if you use one of the higher res mirror textures available ?
If it still looks the same with a higher res mirror texture then it's a mapping issue and in 3DSimED you can try Rescaling the texture. Just make sure to separate all 3 mirror meshes and do one at a time, then when they look right you can group them into one mirror mesh.
As mostly the usual texture is a usual mirror.bmp ... I guess that the used mirror mesh needed to be resized to fit in the mirror posts.
Except resizing them with the same values for X, Y Z ... there's always a bad distorsion.
@Zipdrive
I never tried but in place of resizing a different perfectly working mirror mesh isn't it better to cut a bigger working one at the right size and shape using for example the "Knife" tool in Blender and remapping ? ( when you're not a 3D expert for creating a new one ).
As you seem to be accustomed with modding ... this precision might help when needed.
If an rFactor mod has been converted to GTR2 this type of distortion can happen if the one doing the conversion hasn't included the original rFactor mirror texture (or one of the HD rFactor mirror textures) forcing the mod to use the stock GTR2 mirror texture.
The stock GTR2 mirror texture is 512x256 where as the stock rFactor mirror texture is 1024x64.
Jempy I've never used Blender. I've only used ZModeler and 3DSimED and I've only mapped a few textures in ZModeler as I mostly use 3DSimED for my modding needs. I have done a similar thing to what you mentioned though. Using 3DSimED I've taken a good working mirror mesh/texture and put it into another side mirror frame. After resizing the mirror mesh all I had to do was rescale the mirror texture. Sometimes I've taken the whole side mirror itself and added it to the cockpit.gmt that had mirror issues or just badly modeled mirrors.