Try the following. You’ll be surpised with how accurate and fun this can be!
In a legal and safe way, make a little test run area. Mark out exactly one quarter of a mile (1320 feet). Weigh the car with a full load of fuel and driver (you can do this at a commercial place that weighs trucks, or place four weight scales under each tire simultaneously, or just use a published figure and add the weight of the driver for an estimate). Start an acceleration run through this distance. When you are crossing the end line, note your speed at that point.