I have a 2004 Audi A6 with 17" rims. If I move up to an 18" rim, with this effect anything? speedo, traction control, etc?

 

I JUST switch my 04 2.7T SLine from 17s to 18s and there is a slight difference in how the car deals with rougher roads and bumps. Since I kept the overall radius of the tires the same, the thinner tire does nottake the hits as well. However, when I upgraded I went from the Pirelli P6 to a Goodyear F1 all season. The handling on the car is now remarkably better and smoother. Also, I'm guessing that the shorter, stiffer tire wall reduces lateral motion resulting in a tighter feel.

It was definitely worth the upgrade IMO. The handling and the looks are both improved for a very slight trade off in how the car takes the bumps.

To answer your specific questions, the speedometer should not be impacted if the radius of the tire remains the same. Assuming the wheels fit properly (offset, clearance, etc), I don't see how traction control or anything else would be impacted by the update.

 

I run OEM 17" for the winter and Velocity 18" for the summer, Pirellis vs ContiExtremes. No difference in speedo and they seem to fill the fenders the same. I think the 18" may feel a little rougher as there is not as much sidewall to soak up bumps and pavement irregularities.

 

Yeah you sent me the picture of the velocitys and they look great. At the end of summer i should have mine!

 

If you increasewheel size the sidewall height needs to be reduced accordingly. Usethe handy tire size calculator in the attached link. It will compare the original size with the new tire dimensions and tell you what the speedometer error will be.
http://www.discounttire.com/dtcs/infoTireMath.dos

 

all you need to do is go from a 235/45/17 tire to a 235/40/18 and you will keep overall diameter and revolution per mile within an acceptable percentage, typically under 3%
All tires are not equal in size/shape even within the same 235/40/18 size... I now have Goodyear F1 All Season and they are big, bigger than Khumos for example. They look great, handle real well on dry and wet roads and their sidewalllooks a little ****** b/c it is fairly square tire and it has a stiff tirewall. It also depends on the width of the rim. For example, a 8.5 will stretch it more than a 8" wide rim, which will give you a squarer tire and more tirewall. There is not that much difference b/w my 18s ans 17's, b ut you will lose a little comfort while gaining in handling with a shorter tire wall that flexes less. Especially the Goodyear F1 AS.

 

I want to get 19s ... what tire size to you recommend and what offset do you recommend ?!

 

as i understood it, the 45 in 235/45/17 is for the height correct? so then wouldnt a 235/45/18 be the same height as a 235/45/17? only the inner diameter should change. or am i an idiot, please feel free to point fingers and call names if so

 

no u are right i would need a 235/35/19 but that is a thin *** tire ... maybe some run flats

 

well it doesn't work this way, the tirewall size of 45 is a ratio of the width as in 45% of 235. So as you plus size (i.e. from 17" to 18" wheels), you want to drecrease your ratio # to decrease tire wall size and make up for the bigger diameter rim and therefore keep the overall height and diameter close to stock.

JOETM: if you ahve 235/45/17 stock, I recommend you get 235/35/19 or if you can fit them 245/35/19 to get a little more tirewall. As for offset,I guess it depends on the wheel but ET35 has worked well for me to get the wheels at the corners and clear brakes.