As we probably all know, the speedometer on our cars tends to read a little on the optimistic side. I checked mine against a GPS on a longish freeway drive the other day, and found that 55 MPH on the digital readout in the tach was actually 53 MPH per GPS.

I was running through the readouts on my ScanGauge yesterday evening, and found something interesting. The same 55 MPH on the digital readout showed up as a 53 MPH reading on the SG. Which tells me that the car knows the actual speed that you are traveling (calibrated for the OEM tire sizes and such), and that it is merely the display of the speed that is optimistic.

That tells me that this is a very deliberate design...

 

I think that is pretty interesting! And although I have read a lot on here, I haven't came across this specific info yet.

So are you planning on getting your speedo adjusted? And this deliberate design... do you have a theory behind why they did this? I can't really think of anything other than fooling drivers into thinking that they are going faster than they really are, for safety reasons. Mine is 3mph fast, always.

 

There is not "theory" there is a very specific reason and the quick answer European laws. Again if you were to search for this you would find many threads on the subject.

 

Speedo being optimistic is common, on purpose, and can be turned off with NCS Expert.

The good news is that even optimistic with the speed, the Odometer is still spot on.

 

Wow, thank you all for showing me that I should not reply to a thread that may have been talked about before. I really feel welcome.

 

re: ScanGauge, I remember reading in the setup guide about being able to enter a speed adjustment to account for tire/wheel size, etc.

It's here:

http://www.scangauge.com/wp-content/...SGE_UM_101.pdf

and has text:

The speed indicated by ScanGauge can be adjusted to compensate for changes in tire size, gear changes, tire wear or any other factor that may affect the accuracy of your vehicle’s speedometer.

I remember having to set / reset this for my Miata now running 14" all-seasons: the ScanGauge, speedo, and GPS all had a different opinion on the actual rate of travel. I adjusted the ScanGauge to match the GPS so I'd have a majority vote.

 

I understand that there are reasons behind it, and also laws. I found it interesting that under specific circumstances the car's internal speed reading matched the GPS reading, while what the car showed me was different.

Presumably as the tires wear, both the displayed speed and the internal speed will show faster for the same GPS speed.

 

since I don't live in Europe, I'd appreciate an accurate display.

 

Then you need to find NCSExpert.

 

So would I and on my Gold Wing, I installed a "speedo healer" to do that. I seem to remember reading that federal law required that a speedo COULD NOT read lower than actual speed, so manufacturers make them read higher to keep from possibly violating the law and to keep us from speeding unawares. BTW, mine reads 3mph high at least at highway speeds.