Saw this posted:
Since we have the benefit of modern GPS technology, I am guessing a lot of people drive around with a map app open that shows your GPS calculated speed.

Can you confirm if the above is true in your Porsche ? this is 100% not true in my DD.

What is your experience ?

 

My ‘99 C4 reads 4 mph fast.

 

Vehicles cannot display speeds less than actual speeds, therefore they over-report speeds.

The MFG's are fined if their Speedo's do not meet International laws ...

One such Law is ECE-R39 that states that speedometers can't show speeds less than the actual speed, and can never register more than 110 percent of the true speed, plus 4 kph.
That is -0 to +10%+4kph ........So at a true 50mph speed the speedo can't show less than 52.5 mph or more than 57.5 mph

The formula is 0  (V1 - V2)  0.1 V2 + 4 km/h

To read the full ECE-R39 .... https://unece.org/fileadmin/DAM/tran...18/R039r2e.pdf

 

My 2001 C2 always reads 2-3 MPH fast.

 

Easy test to do is pull up Waze on your phone and drive your 996 or any car. Works the same in my Q7. You can track your speedo and see that its fast in relation to whats reported by Waze.

 

About 3 mph fast.

 

If this sounds strange to some, the US/Canada are not signatories to UNECE 1958 that initiated these standards.
But Germany is. So while a US car may read within +/- of the actual, our German cars always display faster than actual.

 

My two Honda's, a 13 Accord and a 14 Accord, they're speedometers are dead on accurate with a GPS speed checker. Both my Porsche's are about 2 mph over. All the European cars I've owned over the years were all about 2 mph off also.

Interesting about ECE-R39

 

My truck (GM) is within a MPH +/- at 70MPH.

I am really curious if people are actually seeing 75MPH on the speedo at a GPS indicated 70MPH on stock tires as was relayed in the quote in the OP.

Maybe a couple dudes/dudettes could run a test of delta at 10MPH increments from like 20 to 70... Anyone generous to take it on if you could relays the tires (brand and size) you're running that would be helpful.

Here's a good read on the subject from Motor Trend a couple years ago touching on the regulatory stuff and other factors that affect speedo indicated MPH.

https://www.roadandtrack.com/car-cul...ters-accurate/

 

My car reads about 3ish faster than indicated.

 

I have no idea I always have a brisket blocking the speedometer.

 

It seems the obd speed is accurate.

My car with 315/30/18 tires

 

Thanks for reminding me I had speed data from OBD Brad - here's mine with 305/35.

 

2010 Volvo xc90 is 100 % accurate.
2001 GMC dually is 100 % accurate.
1999 996 C4 reads 4-5 mph high .
2002 Boxster S reads 4-5 mph high .

All on stock size wheels and tires . Accuracy was measured via Android phone GPS .

 

My 2003 996 C2 is 4-5 mph over at 80 mph. Seems to be about 5% +/-. When switched over to kph here in Canada, same, approximately 5% deviation happens. I say percentage, rather than a straight speed number, as it does seem (although I’ve not tracked it quantifiably) to change at lower speeds and get closer. This seems to also include new cars, as we just did a trip with two friends, one in a 2017 GTS and another in a 2023 T, all noting similar variances.

My Honda is within 1 kph accurate.