Hey Guys,
My S is incrementally reading slower the faster I go and I can't figure out why. I noticed this with my stock tires of 205/16 front and 225/16 rears, and then when I put on 215 17x8 front and 245 17x9 rear it's +1 mph off. When I'm going 40 mph, I'm actually going 45, 60-I'm actually going 70 so on and so forth. I have a yellow box I haven't installed it yet, but I'm not exactly sure what percentage to set it to because as I go faster, the slower it reads. I changed the vehicle speed sensor and that did nothing. I also swap diff's with another ap1, I believe it was a 02 but with the same amount of miles of my car at the time which was 79k. My car is also lowered so the axels causes vibration from 45-65, and when I turn in neutral i can hear the cv cups or joints making noise. I bought driveshaft spacers for the lowering of couse I haven't installed it yet, but in my opinion I'm not exactly sure if it will help with this problem also. Could it possibly be a electrical problem? Maybe I need a stabilizer? I use to have a vented hood, and left my car out in the rain a lot so maybe some wires got worn. If ANYONE can help me figure this out, much is appreciated, this is a potential problem for tickets and we know how we feel about tickets and cops.

 

You need to install the Yellow Box and then set it by trial and error to get it to read correctly using a GPS to tell you actual speed, just keep adjusting the yellow box until GPS and Speedometer read the same.
As for the vibration, since you have already purchased the spacers, just install them to alleviate the vibration, alternatively, you could just swap the CV buckets from left to right.

 

I was suggested to just pick a highway speed and adjust the yellow box to that speed error. but I'm curious as to what causes this increment of slow reading

 

Anyone have any idea what could cause the speedometer to read slow?