anyway, I can't set my refresh rate to 48hz, my settings won't let me >.< so I though if it is 60hz 30 fps would be smoother than 48 fps in 60hz but it seems that I was wrong >.<

Did you successfully create a custom resolution and then reboot your PC?  Few displays support 48Hz out-of-the-box, hence the need for custom resolutions.

It's because 24 is an exact multiple of 48 but not 60.

However, the difference isn't that big - possibly because 60 is 2.5x of 24 rather than something like 1.5x.  Again, while 24--to->48fps @ 48Hz was smoother, 24--to->60fps @ 60Hz felt "faster", and for some content the latter may actually be preferable.

Compare this  to the likes of 60fps @ 60Hz + BFR (black frame insertion) vs 60--to->90fps @ 90Hz + BFR and 60--to->120fps @ 120Hz + BFR; the smoothest is the 120Hz but the second smoothest is actually the 60Hz - the 90Hz was actually worse than matching the display refreshrate to the video's native framerate.

Though 24fps--to->48fps@48Hz is definitely smoother than 24fps--to->60fps@60Hz.

This sound very weird and i will check it out tomorrow when i get the chance

Is Degrease grid step good to use?

You should only use it if you have extra CPU headroom because the difference is arguably placebo-level.

In case of 60 Hz display (theonepugna's quastion) I see two alternatives:
1. watch 24->60fps at 60Hz with maximum smoothness. Artifacts will be a liitle more noticeable.
2. watch 24->48fps at 48Hz with minimum visibility of artifacts. Smoothness will be a little smaller. But in this case you need to manually add custom mode with refreshrate of 48 Hz.

But then, compared to 60 fps, both of options are certainly not smooth at all )

Though 24fps--to->48fps@48Hz is definitely smoother than 24fps--to->60fps@60Hz.

(interestingly enough however, 24fps--to->60fps@60Hz feels "faster" even though it's also choppier)

But then, compared to 60 fps, both of options are certainly not smooth at all
(Of course, sometimes "double frame rate" is subjectively smooth enough, especially when watching movies film, not animation videos)

And are you absolutely sure your display doesn't allow you to use custom resolutions?  If you aren't on Intel graphics, then you have 3 different video signal types to choose from: "Standard", "Reduced", "Native"; sometimes only one of them will work.  Also sometimes a display can't handle 48Hz but can do 72Hz, or can do 48Hz but can't handle 72Hz.