In theory, GP4 can't see the difference between hi and normal textures; only if texture is named, example
HI_renault_x.tex,
GP4 uses it in hi graphic setting; and if it's named:
renault_x.tex
GP4 will use it in low graphic settings, no matter of their size.
So, if you're running GP4 in hi graphic setting, you'll need hi-res textures (named hi_renault_x.tex). If you downloaded a car which only comes with hi textures, fell free to copy them and just remove HI_
Keep in mind that GP4 uses low res textures on pit monitors, so you should have low-res textures anyway (the ones named renault_x.tex)
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Bojan Tarticchio