How does the Boxster determine it's speed?

[Qmulus]

Originally Posted by KRAM36

I don't put apps on my phone. Google gets enough info off me with just the crap that's loaded into Android phones. I want to figure the percentage of difference on my own and I need to know how the Boxster 986 determines it's speed. Front tire rotation speed, rear tire rotation speed or neither?

In this case he was being a jackass.

I guess this jackass shouldn't bring up the fact that the speedo reads high by design either.

It used to be that speedos got their signal off the output of the transmission, and gears were used to calibrate the speedo to tire size. Now the cluster gets a signal from the ABS and uses a calibration factor (like pulses per mile) to calculate speed.

In the end, it really doesn't matter how the Boxster calculates its speed. If you assume it is accurate now, which it really isn't going back to the first part of this post, all you have to do is calculate the percentage difference in circumference of the new rear tires compared to the originals, just like you would with any speedo.

Now if you want to actually fix the speedo so it reads right, you have to adjust the calibration factor in the cluster to match the tires being used. On some vehicles that can easily be done with diagnostics. Not quite as easy on these.

But what do I know; I'm just a jackass.