Uhh.

Kinda.

If your tire is a larger diameter your speedometer is going to read slightly lower then your actual vehicle speed.

I think that's saying the exact same thing, not "Kinda."

Get a decent GPS system to check your speed. Get the percentage error of the OEM baseline at highway speeds before you start. The whole calculation is a simple ratio, so you can just increase or decrease the overall diameter by any percentage to change your speed reading up or down by that percentage. Mine read a few percent high on the OEM tires and wheels.

You can probably change the tires and rims and choose the size carefully to make it read closer to true that it did from the factory if you care much about it.

One of the better calculators: Tire size calculator
There was another I liked, but I can't seem to find it right now.

I have them in 20s. and as someone mentioned earlier yes they stick out in the front for me. Mine are 20x10 + 20 though.

Running 285/30/20 and 255/30/20. The rolling is a bit smaller than my stock 18s but speedo is basically fine.

Here is a photo. http://a6.sphotos.ak.fbcdn.net/hphot...97627490_n.jpg

That looks amazing!!! :yum:

19" Downside
 

My 2011 370Z came with 19" tires. The ride is harsher than you will experience with 18" wheels. This is because the sidewalls of the larger tires are thinner, hence have less flex.

It's all just for flash; there is no performance or handling benefit to be had from larger wheels. In fact, handling can be degraded. So if you really really like the look do it. Just be aware of the downsides.