Speedometer error

[simbob]

Has speedometer error been discussed here before? If so, can anybody direct me to the thread?

The Christmas purchase of a GPS and the coincidental holiday placement by the local police of a "your speed is" radar sign station in my neighborhood both agree that my speedometer reads a consistent 5 mph faster than actual speed. Although the neighborhood sign was checked against the GPS and the speedometer at speeds from about 10 mph to 40 mph (speed limit 35), the GPS shows the same 5 mph overstatement at speeds up to 80.

I know that most cars register a speed slightly faster than actual, with the raw mph error usually increasing with speed, and I was used to a 10% error on my BMW motorcycles, but 5 mph seems high to me for a relatively recent vintage car, and the raw numerical difference doesn't seem to vary with speed.

My wheels are stock 17" Boxster design, and the tires are the stock size.

Is 5 mph an unusual speedometer overstatement in the Boxster?

Thanks.

Bob

[derb]

Here's one post...
Speedometer accuracy

You can also get vehicle speed in kph through the ac/heater controls. This speed is supposed to be actual speed without the safety margin.
http://www.cb-racing.com/boxster_AC.html
Scroll to 20c.
When in this diagnostic mode the heater controls maintain the last temperature setting.
I believe this only works up to the 2000 model year.
I'm not sure if the dealer can remove the error.

Regards,
mike

[husker boxster]

My old 01S exibited the same 3-4 extra mph as all the other 986s. But only Bmussatti talked about 987s. Do they have the same cushion? I haven't had a chance to encounter a speed check sign and who needs GPS in Omaha? Don't want to assume I have the extra 3-4 mph if it's not there. It's hard enough already keeping it down to a reasonable speed. Any 987 owners have some input?

Originally Posted by husker boxster

My old 01S exibited the same 3-4 extra mph as all the other 986s. But only Bmussatti talked about 987s. Do they have the same cushion? I haven't had a chance to encounter a speed check sign and who needs GPS in Omaha? Don't want to assume I have the extra 3-4 mph if it's not there. It's hard enough already keeping it down to a reasonable speed. Any 987 owners have some input?

My GPS shows that the Boxster is off by about 2 MPH. Same thing for my Ford Explorer. The speedometer reads faster than actual speed.

[tombea]

Mine is also 4MPH fast. This is true for speeds below 50. I haven't found a place I can test it over 50 but I assume it is probably about the same. What I need is a good GPS.

[Lil bastard]

Originally Posted by derb

...You can also get vehicle speed in kph through the ac/heater controls. This speed is supposed to be actual speed without the safety margin.
http://www.cb-racing.com/boxster_AC.html
Scroll to 20c.
When in this diagnostic mode the heater controls maintain the last temperature setting. I believe this only works up to the 2000 model year...

All true. This will give you actual speed, but only in KPH, not MPH, so some math will be needed - KPH / 1.609= MPH (approx.).

Unlike Hondas (mentioned elsewhere in this thread) the Boxster Odo is not affected by this Safety Factor.

[RandallNeighbour]

I thought mine would be higher than actual as well, but when I used my GPS I saw that it's dead on.

Guess I need to tow the line now when I'm around law enforcement. I used to drive a couple of clicks over thinking I had that 4-5 mph safety zone, but I didn't!

[Dr. Kill]

My speedo also agrees with my GPS. I am usually way off from those Police "you are driving XX" signs, but I have always just figured that those things were not well calibrated...

[simbob]

Thanks, Mike.

Bob

[rbruhn]

My 2002 Boxster speedometer is at least 5 mph fast at most speeds, maybe even a little more at high speeds. I just read in the NY Times that Honda has been sued for this same problem because it not only registers your speed incorrectly, it also overstates your mileage and, in effect, shortens your mileage warranty. The Times article
(find it here: http://www.nytimes.com/2008/03/30/automobiles/30MILEAGE.html) goes on to say that German cars are the worst for speedo error, but doesn't mention Porsche specifically. Surely, there's no technological reason for this great an error; after all, it's just a matter of counting and calculating, something that anybody with basic math skills could do. So the overstatement of speed, and hence mileage, must be intentional. Honda avoided prolonged litigation over the problem by offering its customers a 5% increase in mileage before their warranties expire. Anybody think Porsche would do the same?

[Lil bastard]

It's intentional. It's 3-5MPH optimistic. Porsche calls it a Safety Factor.