If you feel compelled to test clibration against a car, have the carmeasure exactly a mile, and then see ifthe bike computeris highor low: don't try to match speed by holding on or otherwise.
But realize car speedometer calibrationis not that accurate. One, tire size (which might notbe OEM, or may be an OEM option not calibrated for) affects it, and Two, manufacturers often purposely make speedomters read a tad high.