mortiz99991

I own a 2001 Boxster s with 42000 miles that I purchased in January. I was suspicious that the speed showing on both the analog and the digital displays were showing a faster speed than what I was really going. A GPS revealed that it was reading about 10% faster than I was actually going.

The rear tires are the same as factory. The front tires are a bit smaller (the previous owner changed them out with smaller tiires). I was under the impression that the speed is monitored from the transmission, so the front tires would have no effect on speed. Does anyone know any diffrent?

dennisafrompa

..German cars and Boxsters in particular are on the high side..my speedo is 4mph high at about 75mph.

1schoir

Here is a technical explanation from a poster on RennTech.org:

"I spoke with a friend familiar with the instrument cluster system.

The cluster controller receives a speed signal, typically CAN data from another vehicle controller, and then converts this via an algorithm into both the digital display value and the pointer position. It appears to be a linear function but it doesn't have to be; for example the graphic scale could be expanded on the lower range, of middle range, or upper range, depending on the manufacturer requirements. In the case of my S, such graphical trickery could be useful as the scale covers 175 mph in a 180 degree sweep and is pretty much useless as a ticket avoidance device. The difference between 35 and 40 is pretty hard to see. And also note that the display and pointer position are hopefully matched to avoid user complaints.

To cover any manufacturing tolerance in the dial graphics, and to insure the legal tolerance range is met (remember that tire model, wear, and inflation all have influence), an offset in the algorithm function can be introduced by the system designers, the goal being a skewed tolerance to make sure you don't get a ticket for going 64 when your speedometer says 60. The instrument controller algorithm is not adjustable by mortals. The odometer may or may not have the same issue as it may be driven by a different algorithm and there is no skewed tolerance concern. Having said that I've checked my odometer on long distance trips and it seems to be plus about 4 miles every 100.

It could be possible to trick the system with different tire size, or by finding the source of the speed signal and assuming it counts pulses from a toothed wheel, add or subtract pulses, but this would only fix a specific speed error at one point because such a 'fix' would introduce a percentage of value error and the speedo would only be accurate at the one point where both the curves cross. For example, if as built it is always ~~4mph high. If I increase the tire size by 6.25%, then at 64mph the indicated speed and actual speed would be equal. However, at 120mph indicated, actual speed would be greater, and at 30mph indicated, actual speed would be lower.

An interesting subject but not an easy fix apparently. "

Regards, Maurice.

mortiz99991

I have heard that they are high, but do the front tires have anything to do with it?

Marc Gelefsky

In have never met a Porsche with accurate Speedo