Today I was dataloging my car and was about to enter a school zone. I looked at my dash and brought it down to 20. I glanced at my laptop and it said 25. After the school zone I took it up to 40 on the dash and yet again 5mph over. I took to the highway, 70 on the dash- 74 on the laptop. It was a bit harder to tell at 80, but it seemed to be closer to 2-3mph off. which is correct?

 

If your at idle and doing 0 MPH, does the laptop say 5?

 

no, its says 0.

 

What about when your speedometer says 5, does the laptop then say 10? If so, have a friend walk next to your car while the speedometer says 5. 5 MPH is a fast walk while 10 is a decent run speed.

 

I haven't tried it at 5, but will try it.

 

What I would do is find a nice suburb neighborhood because somewhere they usually have a speed limit sign that tells you how fast you're actually going. I dont know any places in SA but brodie lane in austin has one about every 2 miles. I dont know if you guys with the vss have a smoother reading than the mechanical speedos but when im driving at a slow speed like 5 or 10 my needle bounces and becomes worthless so the experiment above wouldnt work for me, maybe you high tech guys have better luck.

 

you could also get a buddy with a car (one that you would expect the speedo to function accurately in) to drive beside/behind/in front of you. Then compare readings.

 

Drive down the interstate and hold a constant 60MPH. Measure the time it takes to travel 1 mile (use the mile markers, not the speedometer).

It should take you 1 minute to travel 1 mile at 60MPH.

You can drive at any constant speed and using the mile markers, calculate your speed.

Or find someone with a GPS that measures road speed.