The average speed of a car on an interstate in California can vary depending on various factors such as traffic conditions, speed limits, and the specific interstate highway in question. However, it is common for vehicles on California interstates to travel at speeds ranging from 55 to 75 miles per hour (88 to 120 kilometers per hour). These speed limits are typically enforced, and drivers are expected to adhere to them for safety reasons.
It's important to note that speed limits can vary on different stretches of the interstate, and there may be areas with reduced speed limits due to construction, heavy traffic, or specific regulations. Therefore, it is always advisable to obey posted speed limit signs and adjust driving speeds based on the prevailing conditions.