My gta:Sa was working perfectly but when i modded my game (using san andreas mod installer aka sami) it stopped working properly. I replaced a bloodring banger with a bugatti veyron. The game works OK but it doesn't work fluently, it has some slowdowns. And when I opened SAMI and then clicked on 'un-install a mod' and then clicked clicked on bugatti veyron, it said that it successfully deleted the mod but when i click on spawn the bloodring banger (via 'oldspeeddemon) the bugatti veyron still comes. And when CJ is next to the bugatti veyron, the game starts to have slowdowns.

What should I do?

I would suggest re-installing your game. That's the problem with mods - they can have some undesirable effects. It's a good idea to back up files beforehand.

It's because the new car has a lot more polygons and/or textures than normal cars, and your PC is struggling with it.

so what if i replace the car with a differnce car?

If the car you want to replace with contains less polygons ( Less detailed and smooth) then its okay.

is there a gta mod website that tells you that it doesn't contain any polygons?

What do you mean dude? Every solid object in San Andreas (or any game, for that natter) is made out of polygons. The more polygons, the more detailed the structure is, but a more powerful computer is needed to handle it.

what i meant is that are there any mods that tells you beforehand that this mod doesn't contain that much polygons?

your computer is just slow not the game.

Un-install the game then install it again!
This also happened to me!
REASON IS- YOUR computer is over-heated
or- your computer doesn't mean the requirements