MPH Issue Revisited

[PaulE]

That's interesting. What size tires do you have on the 17's and 18's? I've got a set of 17's and a set of 18's for my 2003 S and the speedometer is off about the same with either set on the car. My tires are all Michelins in the OEM sizes, 205/50-17 and 255/40-17 and 225/40-18 and 265/35-18. And even with the same nominal tire size, the actual height and width of tires will vary by manufacturer.

[freserf]

I like the Cuban missile input ... will probably be able to track speed more accurately once Musk finishes Starlink. Any guy who can launch his own car into orbit doesn't really care about speedometers I guess ...
Anyways, I only began to research my speedo issue because my wife was following me to lunch one day and she asked why I was driving so slow. She asked if I was trying to piss off the guy behind me because he was up in my exhaust. Well, that is when I typed in the question on DuckDuckGo and to my surprise, the speedo issue has been around since the beginning and because I am only 5 months into my first Porsche project ... I am still learning all of the good and the bads of Boxster ownership. I will probably stick to the same size 17's on mine when I shoe on some new ones. These tires are 6+ years old now with good tread, but they are showing signs of age. I will be looking at the same Michelin, Continental or Bridgestone ... not sure if I will purchase summer tires or all-season.

Also, I will be looking into a more accurate speed readout ... If I can modify the pulse signal slightly, it should give me the ability to correct the error. Maybe if I find the time.