I don't know how a car gets its speedometer to register its speed, but I do know overall tire diameter can cause false readings.

So, what's different between Z's with 17" wheels and Z's with 18"wheels?

My goal is to put 18's on my Enthusiast, while trying to remain as close to the original overal diameter as possible while at the same time trying to have enough tire so as to not crack a rim on potholes, etc. AND still keep my speedo somewhat accurate.

What part is responsible for the speed reading and is it interchangeable between models?

 

I'm thinking about doing this as well. I know with 18's I'll actually be going further and faster than the speedo and odo are telling me. Can you get the car's computer fixed to incorporate this change?

 

If they're the same than the overall diameter is the same between 17" and 18" models, the only difference being more diameter is wheel vs. tire.

Here's a website with a tire calculator that estimates you how far off your speedo will be between stock & different tire/wheel combos: Tire calculator