My 99 Boxster's speedometer was consistently 4mph over that detected by radar.
It this also true for the 997?

 

I don't have the technical answer, but I think the general rap is that Porsche speedos typically show a little higher than actual speed.

 

My 997 speedo is right on

Confirmed with a portable GPS

 

Aren't most radar guns calibrated 4-7 mph under actual speed?

 

The best method is as stated above,--using a portable GPS receiver.

Several years ago the US DOT (transportation dept) forced manufacturers to intentionally lie to car owners about speed. They forced them to have them err toward to high side,--making us all slow down. That requirement was dropped when the 85mph speedo marking restrictions were taken away.

dan

 

My 2005 C2S reads 5 km/hr too high at speeds of 100 to 140 km/hr. When speedo reads 160km/hr (100mph) Garmin GPS true speed of 152 km/hr. So either my Garmin GPS is wrong, or the speedo is optimistic.

FWIW, at 50km/hr ~30mph, they both read the same.

 

My 2004 GT3 reads 3km/h more than the actual speed.
At around 200km/h, the gap increases to about 10km/h.

 

no.

it is a well documented fact that many modern cars, especially high performance ones, speedos go over slightly.

many porsche owners report 3~mph speedometer deviations over 20-30~mph and the margin for error stretches even higher at higher speeds.

 

I guess I'd like to know how my speedo compares to a policeman's radar gun.

 

Yesterday with street radar box (what do you call those things anyway?) my indicated 42 was read as 38 Thats a 10% error! Should we test 0-66?

 

I do not have my 997 yet but the 01 BoxsterS my wife had was off by around 3 mph if I remember correct. My 2005 BMW X5 and my wifes 03 BMW 740i is off by almost 5mph. I have a Garmin GPS I checked speeds with.

 

Did you find a fixed 5mph error or a percentage which increases with speed?

 

At 122 km/h reading, the actual speed is 120 km/h, which is pretty close.

For people living in Europe it is an easy check: set your speed at 120 km/h with cruise control and count 100 m signs along the highway, and clock with your Chrono. If it takes a little more than 30 seconds, you put the cruise control at 121 km/h and try again. When you count 1000 m (10 signs of 100m) in exactly 30 second you drive 120 km/h. Mine hit that spot at 122 km/h.

 

Car speedometers are NOT precision instruments...not in consumer cars. Some may be more accurate than others, but - generally - a dead-on, accurate speedo is pretty rare.

 

Simply verify speed by calculating based on engine speed, gear and tire sizes. This calculation will get you withing 1% of actual speed. Many have changed tire sizes and have greatly changed the accuracy of the speedo. If you go to larger tires, your speed will increase compared to the speedo reading and smaller tires will make your speed slower than the speedo reading. Tire air pressure wil also effect the accuracy.