acao

In case anybody is wondering, I had the opportunity to observe the indicated speed as shown by three sources: speedometer, OBD, and GPS. The OBD and GPS signals came from my Aim Solo DL.

OBD and GPS show the same speed value. The speedometer displays a higher value. The delta varies, but at 65 mph on the GPS and OBD, the speedometer shows 68.

The bottom line? The car has a more faithful measure of speed, but Porsche has chosen to not display it on the dash.

Fred R. C4S

Porsche didn't choose this on their own. It's mandated by several government bodies. BMW is often off by a fixed offset and a variable amount than can total nearly 10%. Do a Google search and you can find more details. The bottom line is that governments want never want a driver using the speedometer to be over the posted limits.

ecostellodo

Same here. My car and my son's BMW each say 3 mph faster than the true speed. My GMC yukon is spot on.

simsgw

Yep. Regulatory thing. The basic system is amazing actually. I've tested it against radars and it's always two mph fast whether the tires are well worn or brand new with thick tread. Obviously, the computer just tacks on two to meet the rule.

Gary

simsgw

Just curious, but assuming that by OBD you mean On-Board Display, then why did you need the Aim Solo to get that value? It's displayed on the dashboard in digital form as well as the analog version I assume you meant by 'speedometer'. Or isn't it? Is that just a dot two choice and the dot one leaves it off?

Gary

acao

No, not on board display.

Marshal_Mercer

OBD = On-Board Diagnostics

Jake951

My GT3 speedometer reads a constant 3 mph high from around 15 mph up to about 70 mph. However, above that the difference starts to grow and it's off by about 5 mph above 100 mph.

Mumbles

So your warranty will expire before it "truly" expires ?

Edgy01

Truly, Porsche knows how fast the car is going because the rear spoiler deploys at 75 mph--which is NOT 75 mph INDICATED. It's more like 77 and a little bit for my particular car. Personally, I'm sick of the government mandating stuff like this, because I have to mentally subtract 2 mph every time I look at the digital readout to know what I'm REALLY doing. Years ago, the DOT mandated a even larger speed delta (always speed optimistic).

Yet anothercrisis

All the discussion about regulations is interesting. I had never heard this. In fact, I considered my last car (a Corvette C6) "dead on." The digital reading was within 1/2 mile per hour of the analog readout (digital did not read in less than whole numbers so the speed jumped to the next number after 1/2), both of which matched the GPS, and all matched law enforcement radar. At 55, 65, and 75 MPH the numbers matched exactly. Maybe my measuring devices are all suspect.

Edgy01

When I learned about this fudge factor I did some reading. Go look it up on the internet (the all knowing). DOT controls everything.

All Regulations
Part 393

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Subpart G - Miscellaneous parts and accessories

§ 393.82Speedometer.
Each bus, truck, and truck-tractor must be equipped with a speedometer indicating vehicle speed in miles per hour and/or kilometers per hour. The speedometer must be accurate to within plus or minus 8 km/hr (5 mph) at a speed of 80 km/hr (50 mph).
[70 FR 48054, Aug. 15, 2005]

DHC

My Carrera is significantly more accurate than my previous BMW. Not even close. Still a couple MsPH off, but not as annoying as 4-5.

gota911

The speedometer on my 2010 C2S shows 2 MPH faster than the actual speed.

Minok

Thats not about a supposed fudge-factor, that is about the required accuracy of the speedometer. All that reg says is that the speedometer needs to be within a certain tolerance at 50mph.

It just mandates the quality of the speedo be at least this good.