I've had to delete and update the merc skin post, it was wrongly named so everyone would have instead downloaded it off LFSW which defeats the point of this thread.
I believe thats the correct ratio to acheive 1:1 pixels on the cars.
Not really sure why its the case, but on a 1:1 skin (say 2048x2048) the width gets stretched to cover the car, so having a single pixel on the nose of the car would cover ~1x1.1 pixel, you'd get a slight blur on the next pixel. You probably wouldnt know it unless you'd done a fair bit of skinning and tried to line things up perfectly, but the best you can do is have a perfectly sharp line join to a half-arsed one on the other section, this is because its stretching the jpg's pixel.
2560x2048 would give you square pixels, 1:1 on the car.
As i say not sure the specific reason LFS didnt go with that, you dont buy a medium sized T-Shirt if your a XL fit, and expect it not to be stretched out of shape, you'd go with what fits, and LFS's has square pegs for a rectangular hole.
It'd certainly help with file compression (only cos theres less pixels to store), but if thats the real reason its a joke considering its LFS, if the same shortcuts were taken for the game engine none of us would be playing it.