While I was working on one of my cars that I'm releasing soon, has the same problem that my Xj220 has, the car looked all horrible looking and stuff, and wanted to figure out why, so I've spent the last few hours uncovering that issue, and I've found out that importing a mesh using the GMDC does in fact have a con to it, it merges a certain amount of vertices of the mesh, which results in loss of quality, but it's only noticeable on certain meshes and some others will end looking like my Xj220 and it did happen to a GTA IV car, so here's the fix, after I've imported the new mesh into the SimPE file, I export through the Asci II text file of that mesh back into MS3D, then the mesh that I still have in MS3D I separate the body, wheel and doors, so it can be re-assigned, and I export that file in 3ds, then with the other ms3d I have open with the imported acsi II mesh, and by looking at it, I could tell that something happened to it, so I delete all object groups and I import the unaffected mesh that I put in a 3ds file into this, reassign everything and put the car back together, export using the asci II tool and overwritten the old file, and import the new asci II file, and the mesh will have it's full detail in-game, the ACSI II tool was used on my old cars before I learned how to adjust the animated joints for the file and I think F-P still uses this method, would also explain why he never had this problem and his cars look very clean and smooth in-game , and also explains why this issues is only recently affecting my cars, but the only con to this method is that the mesh will have all it's vertices in the file, so the car's overall vertex count will be higher, I don't think it will affect game or not for those people who have had issues with my cars showing up correctly in-game, but they work much differently than faces do in polygons(polys), this only adds an extra 5-10 minuets to my car creation, but this little extra to do does a lot

If you don't have a clue what I just said
Here is a before/after images of my XJ220 with my method

Before-


After-


The car in the back is the other car that had this issue, it was also fixed with the same method, I didn't take a pic of the car when it had the issue, only after I've fixed it is in the pics, but starting with that car, I will add these few extra steps to all of my future cars and I will update my Xj220 shortly, I think I should make a new vid, since the car looks completely different now and I'll post the update later today or tomorrow, as it's almost midnight here lol

Great job! It does look alot better with the new method.
btw Sentinel XS looks great

Wow, that's a huge difference!

So good you found out about it!

Nothing to be with the old, this jaguar is totally best than the other. You are great Vovi

So much better!

It took me a while to figure out what you were saying but now I get it!

could you try this with the opel zafira???

this trick is already applied to all my conversions, starting with that Jag

lol i'll be the jugde of that jk