If you are trying to speed up the grinding process, always have a flag mounted on your car. You can only get purple engineer flags(+100% exp) from boxes. The blue ones (+25%exp) can be manufactured at the white engineer workbench for 300 scrap and 150 copper. Flags can not be traded(sold). It helps to have a party full of people with flags because each one will give a 10% exp boost to everyone else as well (15% or 90% to you alone based on the color, 10% to your team).
Purchasing the scanner also helps, it gives a 50% exp bonus as well.
There are various items called appearance items that give up to 4% more exp per item up to a total of 25%.
Raids can give up 14k exp with all the extras loaded on and if you perform well. Normal mission battles can net ~10k under the same conditions.