It is possible to upgrade four components on each vehicle:
- Speed - go faster
- Grip - better traction to help you up hills
- Stability - less flippy and easier to control
- Tyres (differs for each vehicle) - improves traction on the stated surface type
Upgrade them by spending truck currency - which you collect in races. If you don't have enough truck currency you can use gems instead.
Alternatively you can sometimes watch an advert to upgrade for free.
Once a vehicle is fully upgraded, you are ready to import the next, improved model - see 'What does import do?' for more information.