Hello in this guide i`ll show you how to change cars in GTA San Andreas Single player.
1st you need IMG Tool.You can download it from here
2nd Lets take a a new car.Choose one from here: http://www.gtagaming.com/downloads/gta-san-andreas/vehicle-mods
3rd Extract the dff and the txd of the car somewhere.. it doesn`t matter.Also don`t forget to readme.txt
4th Start IMG Tool 2.0.Press file or CTRL+L and go to ...../GTA San Andreas/models and find the file gta3.Double click on it and then press F2 and write the name of the car you downloaded.For ex. you have downloaded BMW M5 with name admiral.Press F2 And write admiral,then delete Admiral.dff and Admiral.txd.After that press CTRL + A and find the new admiral.dff and add it.Do the same with admiral.txd.Then open readme.txt and read carefully what it writes.
5th Files in most cases you need:
carcols.dat,carmods.dat,vehicles.ide,handling.cfg which all can be found at ..../GTA San Andreas/data/
6th replace the numbers from readme.txt (that one which you have downloaded with newcar.dff,newcar.txd) to those files which i have counted in 5th step.
7th save everything and start gta_sa.exe
~Have fun
If you are having any problems you can always ask :)