PAULSPEED

Hi Guys,
I put new batteries in my hand help GPS. I was driving at 75mph and the gps said I was doing 72.1mph. What gives? Do the tires cause this. What way does these cars measure speed? Has anyone else noticed this?
Paul

Cope

I would say that the tire diameter is most likely the main contributor for the discrepancy you are seeing. There is no way for Porsche to know exacty what the diameter of the tires on your car will be at any given time, so they have to make an educated guess when they setup the speedometer's calibration. Tire diameter is effected by pressure, wear, the variation inherent in the process by which the tire was manufactured, etc. There could also be some error from the rotation sensor itself (I'm guessing that the car has a rotation sensor, like many of today's cars do) but most likely this error isn't as significant as the tire variation.

Fahrer

Although speedometers are generally much more accurate than they were in the past, my guess is that you simply have a speedometer that is a bit optimistic ( not too uncommon). Also, were you driving for some time ( 60 seconds or so) at a constant speed while reading the GPS? GPS is not that quick or responsive compared with a speedometer.

Ian C

The gps speed is the more accurate one if you were driving in a straight line. Car speedos are simply not as accurate.

helispud

I have installed a set of Yokohama Advan S-4 all-seasons for the winter. Even though they are the same size as the PS2's you can see they are smallerin diameter. In checking both tires specs the Yoko's are about 1 inch smaller in diameter. However, now when the Passport 9500i goes off it's speed readout is only 1 mph slower than what the speedo says rather than 4 mph with the PS2's. At around 40 mph.

PAULSPEED

Hi,
On another forum I was told that this is very common on 997 cars. I still find this hard to believe.
Paul

Likemystoppie?

You're possibly traveling between two dimensions. Or outrunning the Satellites.

p-cardriver

On the BMW forums they attribute this (very common) to German/Euro laws that severely penalize mfgrs if the speedos read too low but give a wide margin if they read too high. So, the theory goes that they are all set up to read too high usually by 5-10%.

Don't know if this is true or applicable to p-cars, but it makes some sense.

p-cardriver

Crazy Canuck

Both my cars read high by about 5%

dndodd

Versus my Garmin 2800 series my speedo is only optimistic by 1 or 2 MPH. Stock Ps2's in the front 305's in the rear.

spiderv6

That is true.

CButler

The difference between my speedo readout and my GPS-equipped 9500i readout is 2 mph. If I set the cruise control on 81 mph, I'm actually only going 79 mph. I think this is pretty typical for most all cars, Porsche included. Think of it as a safety...like setting your alarm clock 5 minutes too fast.

Stephenkng

Should we receive an additional 5% mileage for our warrenty as well??????

KeninBlaine

I believe it has been mentioned before on this forum that Porsche 997 speedos read high. I have found the same on mine. That is one reason I am considering putting 305/30x19 on my new 11.5" Champion rims, as the slight increase in diameter/circumference from the stock 295/30x19 will help reduce the error. I agree that the Garmin is likely the best way to check. It is also handy to track top speed. I busted my son when he took a trip in my C55 to Vegas. When he returned I checked the max speed on the Garmin and busted him for doing 105 on the trip. Next time he'll wipe it clean before returning I'm sure. Fun while it lasted.

Edgy01

Many years ago DOT mandated that speedos read out very optimistically,--all part of an attempt to slow us down. (That failed). But they still dictate that speedos should error on the optimistic side. When I am going about 70 mine reads almost 73. Didn't bother with GPS to do it,--just judging from all the radar-guided speed signs around ("your speed XXX")