Re: Speedometer Error

I recently checked my speedometer (1998 Boxster with correct size tires on factory 17" wheels) against my new Garmin GPS. When the Garmin reads 70mph my speedometer reads 73-74mph. Assuming the Garmin is the more accurate, is this speedometer error common with the Boxster?

all german cars i have ever owned read slightly higher than actual speed. my BMWs read 107 KPH at 100 KPH actual. Boxster was about 4 or 5 over.
it is an intentional buffer so that you're always a bit under the limit since speed measured from rubber tires turning can not be exactly right.

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MY 2000 S, Ocean Blue, Metropol Blue, Savanah Beige.
Bought June 2000 - Sold May 2010

speeds as well say around 4mph high at 35mph and maybe 6mph at higher than well higher than 70mph speeds).

Been that way from day 1. Shortly after I bought the car new in 02 I used a Garmin Etrex (IIRC) to check the car's odometer and found a difference of just 0.2 miles over 200 miles between the car's trip odometer and the trip distance traveled readout of the Garmin unit.

Since then I have also checked the car's speedo with a Garmin nuvii NAV unit and by monitoring the speed obtained using a tool connected to the car's OBD2 port, the speeds from the latter two agree with each other but disagree with the car's speedo.

In short, normal. Some Porsches have a bit less error (2mph for instance) but that the speedo reads high is normal.

Sincerely,

MarcW.

Our '06' is 2-3mph high at 70; our '99' was +5mph. somewhat better with the newer car.

Like others have said, I think it's normal. My 06 is reads about 2 mph high at 60 mph when compared to my GPS.

My '09 reads only 3 a 4 kph high at 100 kph.
It's an improvement compared with my '99 which was about 7 kph high at the same speed.
I hate instruments purposely reading wrong, I know the reasons but to me they are patronizing.

My '99's speedometer reads a consistent 3mph high, GPS and "Your Speed Is" signs read 27 when the speedo reads 30, 67 when the speedo reads 70. Most countries require a buffer to ensure they never under-report speed: Wikipedia - Speedometer Readings - International Agreements

The early Boxsters were about +5mph high. A software update from the dealer changed that to about +3mph.
I got the software update and mine is now about +2-3mph high versus GPS and electronic radar signs.
If you don't have the on board computer that dealer software update will also permit you to turn on the temperature display as all early Boxsters had the front temp prob installed. Mine was turned on by the dealer.

Thanks to all for your inputs.