996 Speedometer Calibration

I know this topic has been covered many times, but most of the threads are 2008 or earlier. Since then has anyone figured out how to calibrate the speedometer so it doesn't read high all the time? I had originally thought it was just my winter tires (Boxster size) throwing my speedo off, but I have since had about 3k miles on my summer tires (stock 18" size) showing the same exactly 4 mph high at all speeds from 10 mph all the way up through 80+ mph. i.e. 69mph on the speedo is really 65 mph. It probably is not that big of a deal, but it is a pet peeve of mine and drives me absolutely crazy, so if it is in any way possible to fix this I would love to. Any help would be greatly appreciated. Bonus Question: Does anyone know what temperature the third white line on the temp gauge represents (the one between the 180 and the red)? The gauge clearly isn't linear and I couldn't seem to find the answer to that in the search.

You are going to have to chill on this one. The speedometer will always read 4mph high, and there is nothing you can do about it. There is no "calibration" technique. Mine does it and I don't care. If it drives you absolutely crazy and you cannot live with it sell the car. I'm guessing on the temp gauge line..225?

Quotes source post: 18200080

That�s about 100C in my car, a 99c2 cab. I don�t think it�s the same for all model years.

Think about what you save in speeding tickets. Think your going 80 in a 65, only 76. That’s a big difference in $ and points.

Bigger tires

Quotes source posts: 18200249, 18200340, 18200377, 18200465

While I find it very annoying, I definitely get more enjoyment out of the car than frustration from this, so I will definitely live with it if I have to. Thanks, mine is a 99 C2 also, so it should be about the same. True, but I'd still rather have it be accurate. I will definitely look into this in a couple years when I need a new set of tires, but I am guessing the MPS4S that will fit on my RS-GTs properly will all be about the same diameter and not solve the issue.

Somewhere I read that the manufacturers of euro cars are required to have the speedometer show a higher speed than what you are actually going. There has been a number of threads and comments on this subject. Maybe do a search in this forum. Or a Google search. In other words, Porsche engineered our cars to meet that requirement.

Quotes source post: 18200992

I read that too in my searches but that seemed really odd to me since none of my other German cars ever read off like this. My 2014 C300 was dead on, my 944 was dead on, but someone definitely did calibrate that speedo for much larger wheels/tires, and all of my old German cars were pretty spot on 60s/70s Opels and VWs. Even if that is how it was set from the factory it just seems odd to me that they didn't put any provision for calibration. I miss just being able to swap a gear to get your speedo right, although mechanical speedos definitely had their own slew of problems.

Its some EU regulation where the speedo absolutely cant read under your actual speed, so they put it a bit over to make sure that doesn't happen. Since you have a '99 you can use 6C in the hidden AC menu to get an actual number for the coolant. I've found the dial to be decently faithful, I think the line is ~225F https://rennlist.com/forums/996-foru...ml#post7057458

Quotes source post: 7057458

The problem on many of the 99�s is that the HVAC hack shows 117C, while the DME shows 100C when I check via Durametric (also verified via my ODBII app). The reason I know this is because many people complained about their fans not coming on when the temp gets well past the 180/200 mark, into the 225F mark, yet the DME controls this activation. Same with my car. It turns out, my car is behaving properly, at least according to the DME. The other anecdotal proof for me is that 117C is super hot (242.6F), and I highly doubt my coolant is that hot.

Quotes source post: 18201023

I�ve had BMWs that read a lot higher, as much as 7mph higher. Our Audi Q5 was dead-on, and I got more cops to pull me over with the Audi than the other cars.

Huh, I think I checked my obdII scanner at the same time and saw agreement, but don't recall. According to the post below the sensor is actually 2 sensors, so it is possible to get different reading between the two, wild. https://rennlist.com/forums/996-foru...l#post16898066

I have a Durametric and an OBD II scanner, I didn't even think to use that when the car was hot to check the gauge temp. I will have to test that and the 99 hack and see how they compare. Thanks for sharing that hack dsbl.

Quotes source post: 18201117

Our Q5 was dead-on also. The Macan that replaced it reads a bit high. It's odd that they would be different given that the German law should apply to both Audi and Porsche. The 996 reads high, those I've got to check it again after just putting on new Michelin all-seasons.