Tachometer vs. GPS accuracy

It is always to hear, that tachometer in the car shows a bit slower speed than reality. But how slower ? An easy way to find out is to use the GPS. I did it, using the Saab PhoeniX Android application on the HTC Desire smartphone. Since the GPS often "could not decide" the speed, jumping +/- 1km, this is a best try at longer routes and using multiple measurements. Vehicle : Saab 9-3 convertible.

The results :

kmh		Mph (US)
Tachometer	GPS	Tachometer	GPS
50	49	31	30
60	58	37	36
70	68	44	42
80	77	50	48
90	87	56	54
100	97	62	60
110	107	68	67
120	116	75	72
130	126	81	78
140	136	87	85
150	146	93	91

As you can see, the accuracy of the tachometer go slightly down with the increasing speed against GPS. Still this is the most accurate tachometer I ever had. Please be aware, even if GPS is now the most accurate measurement technology widely available, there are differencies in  curves and the police radars are using different technology to measure your speed. Therefore with this I dont want to say, you should speed. It's your decision how fast you drive. Km were converted to Miles through this tool. 

From a trip to Prague I wish you a safe and relaxed ride on Saabs wherever you go.