TUTORIAL on shrinking GTA DFF's file-sizes and optimizing their polycounts (better for slower PC's, low poly = less FPS lag and GPU load). I noticed there aint much good guides like that out here, while people are asking how to but only get generic answers like, ''Reduce poly count'', well here is a complete step-by-step.
Note: in this whole tutorial I use vehicle atomics (like ''chassis'') to explain, but you can also use the main de-polycounter method for any kind of .dff like skin, gun, map part mods. Just likewise, start with the atomic (material in list to left) that has the greatest amount of polycount (vertexes) and work down to atomics with less poly, to see the big size effect quicker.
1) use 3DS Max
2) import the DFF with 3DS gta plugin (like Kams GTA scripts)
3) from the atomic list, start first selecting the chassis (named similarly) as that usually covers most of the model polygons, then unfold it so the dummy of chassis reveals its components, make sure then to select the real chassis (on the view in right tab it will no longer indicate its a dummy if you do it right) and click in 3DS the square-window icon and find the toolset you wanna use to reduce the poly
4) From the last step I advise you use ''MultiRes'' (other stuff that I don't prefer much you can use is like ''ProOptimizer 3DS tool)
5) so now you got the Chassis atomic selected after importing that DFF and you selected through dummys to actual chassis (in this case, the 3DS viewport will show a major part of the model ''lines'' light up and select, as I said chassis covers a big part of whole model, obviously)
6) Start reducing polys. In the opened MultiRes tool, first click ''generate'' and choose a percentage to keep from the original vertexes/polys, like you can reduce it to for example 70%, you must hit Enter after customizing this value, then after hitting Enter you click ''generate''. You will see the model image (3DS viewport) change as the polycount is being reduced.
Now you gotta CTRL + A or Select All atomics from the list on the left, then use the 3DS Max GTA DFF exporter of your choice (like also Kams Scripts.. or another) to export the new DFF with your updated polycount/vertexes.
The tricky part: how far you can go with reducing polys depends on the model itself, with some mods it can easily break the looks and bring down a HQ car to low quality SD. You will need to repeat this process many times (export DFF and test in GTA or MTA after changing the value of % vertexes reduce in MultiRes) until you find the perfect polys count to bring it down with.
I personally use MTA:SA with a bind on test server '' /bind 1 restart carmod '' to restart and reload the car without closing SA, so I can update the polys, > export new DFF (into carmod resource folder in MTA server location) and test new value if it looks differently, in less than 20 seconds. If you have to use GTA SA you need to restart GTA each time to load the new .dff mod into GTA3.IMG which takes much more time up in testing the new looks. The 3DS viewport image does NOT accurately represent the new quality, you really gotta look at the new DFF file in GTA/MTA itself. You also cannot create a 3DS render for GTA DFF's preview.
Even more so, with certain models you will find graphical glitches when you really go too far (eventually with any model) which means there'll be gaps in your car/vehicle mod. A short term fix for this is not only ''generating'' in MultiRes with a reduced % or vertex count, but ALSO selecting ''VERTEX MERGING'' checkbox underneath it, before hitting Enter + Generate and exporting.
Vertex merging tries to cover up for these gaps by connecting 2 mesh parts that otherwise would leave an big open gap because they were shrinked. This will however have a downside too, do it too intensively and your mod textures will become blocky (like say bye to smooth corners and sleek, awesome looking surfaces) and after all if you rely too much on it, a point will come vertex merging cannot even handle it anymore (that gaps and holes in the model will anyhow appear - like say ur trying to convert a 60,000 poly car into a 2000 poly car - common sense tells this simply will not work and the possibilites arent infinite - there WILL be a breaking point.) however this breaking point can be further pushed away by ALSO enabling ''vertex merging - > Within mesh and Keep boundaries. But as before, even after enabling this, the whole Vertex merging ability plus these 2 other settings will have a breaking point so dude I wish you luck converting a 12 MB vehicle mod to your desired ''300kb'', that aint gonna cut the possibilities. You will end up with a cheesecake model despite the improvements such as Vertex merge.
NOTE: I only covered about the ''chassis'' atomic, but just as its a major model component - you can recude polys of any non-dummy material (atomic) which most likely will be stuff such as seats, interior, wheels (if custom and not SA wheels), bonnets, trunks, custom tuning parts integrated to model, ETCETERA..
You can also choose to cut less of the chassis if you can already get a fair enough size reduction you're lookin' for, by only shrinking polys of the other parts and atomics that are NOT the chassis itself. However note that if this reduces size fairly, the chassis can still make up the bigger part where DFF size comes from, and not optimizing the chassis can after all let the mod lag/load the PC you run the game on, the same as original mod, eventho size was reduced.
Have any questions ask me!