Hi recently purchased a new 8 and getting ready for Canadian Winter
I have a set of 16" rims that I want to use but I was wondering will it affact things like the Engine Management or Speedometer?

And does the 04 have a Tire pressure Sensor?

Thanks

 

maybe and yes

 

by a noticable margin? say 2-5km/h? or something like 10-15km

 

depends on the overall diameter of the tire.

 

It might because I have 20"s and my speedometer is one mph faster than I'm actually going.

 

If you keep the overall diameter of the tires within + - .4%, the differance to the speedo shouldn't be enough to worry about. Your real problem is the size of the front brake disk. I'm not sure about the 04's but the newer models that come with 18's have a bigger front disk than the back. If thats the case then the 16's won't fit. Of course you can always try a test fit with one wheel to make sure.
Do you know if the bolt pattern and offset of the 16's will work with your car?