If you are travelling through Europe with a car, one essential thing, which you should know about your problem-free car ride, is that you should have a valid vignette for using the road network of the country you are travelling in.
As the vignettes are requested when you are stopped by a traffic police officer, you should be able to present it in order to avoid getting an undesider fine.
One very simple way to have all the necessary documentation is to stop at the first major brand gas station and to ask about the necessary vignettes in the country. You should also ask if there is additional payment requested for using the highway system in the country, as it might have to be paid separately.
The positive site of using known gas station chains is that you will be able to pay via credit card if you still have no local currency (some EU countries still use their own currency and not the EUR).
Vignettes can be used for a single day or for a week and when buying one you should consider if you will pass trough the country again on your way back.