Accuracy in pinpointing vehicle location depends on a number of factors, right from the GPS device model installed to map data and even to the no. of satellites monitoring that vehicle. Some GPS devices offer location with a predicted error margin of +/-5 meters whereas some of the latest devices available in market offer an accuracy with less than 2m error margin.
Map data plays a vital role in returning accurate locations too. Frequently updated maps bring you more precise location data.
Out of the 24 active GPS satellites orbiting the earth, your vehicle must be in the line of sight of at least 4 satellites with no obstructions whatsoever, so you could get the most precise geographical coordinates (latitude & longitude values). And this would be so most of the times too, unless in certain occasions like thick clouds, heavy fogs or sensitive areas with jammers.
A few factors affecting the smooth reception of GPS signals include:
Underground parking facilities or parking garages
Tall lush trees
Being in a valley or surrounded by hills
Being in a tunnel