It's rather confusing, but all you need to know is that you must have fully completed every objective below for 100%.
Special thanks to my good friend Geordie Dude for assisting me in the creation of this list.
For example Chiliad Challenge 1 gives you no extra percent, neither does the second or third on their own, however after winning all three races you'll be awarded with the percentage. You get no percent for each single car, but you do get percent for the fully completed list.
If you wrote down your percentages after every single objective (as I did) you'll notice the percentage jumps from 0.53% for one objective to 0.54% for the next. What is programmed is that you receive 0.534759358% per objective which is then rounded up or down to two decimal places.
There are 187 objectives in total in GTA San Andreas which are REQUIRED for 100% completion.