RPM/Gear & actual road speed

[husker boxster]

The 3-4 mph "safety factor" has been discussed about 986s, but is it also built into 987s? I haven't had my new one long enough to discover on my own. I live in Omaha and who needs GPS in Omaha? I'd assume Porsche built the "safety factor" into 987s but varification here would be better than finding out via law enforcement that it's not.

[fatmike]

I'm more precise than subtracting 4 MPH (or so). Drives me nuts. I want to set my cruise control at certain places on the highway at very precise speeds. For example in certain spots -- 73 is OK, 76 is not.

Maybe someone with a Garmin can post their speed at 3K RPMs in 3rd gear? (We'd need to know "S" or not & also your tire size). How fast is 3K RPM in 6th? Anyone?

[derb]

If you are good at converting kph to mph...

To access the AC diagnostics:
Hold down the Recirculating & Air Up buttons for 5 seconds. The + - buttons go up and down through the list of "c" codes. The center vent button switches the display between the "c" code and its actual value. Press the Auto button to exit.

Go to 20c.
20c - Vehicle speed in kph, updating only once per second. (real speed, not speed+safety margin as in the speedometer)

I haven't verified the speed with a GPS yet but it does read lower than the speedometer.

[Topless]

If you really want accurate you will need to beg, borrow or steal a GPS. Anything else is loose guesswork.

[Lil bastard]

Originally Posted by derb

If you are good at converting kph to mph...

To access the AC diagnostics:
Hold down the Recirculating & Air Up buttons for 5 seconds. The + - buttons go up and down through the list of "c" codes. The center vent button switches the display between the "c" code and its actual value. Press the Auto button to exit.

Go to 20c.
20c - Vehicle speed in kph, updating only once per second. (real speed, not speed+safety margin as in the speedometer)

I haven't verified the speed with a GPS yet but it does read lower than the speedometer.

Yes, this works. It is actual speed in KPH, not MPH.

To convert KPH to MPH, you use the formula:

KPH / 1.609344 = MPH

But, this only works on cars through the 2000 model year only. These diagnostics cannot be accessed on later cars.