Guys, when running 24" slicks, how does this affect the speedometer reading?

 

speedometer readings are affected by the changein height of the entire wheel tire combo from stock. The change in height effects the circumfrence of the tire. that effects the distance the car will travel per revolution of the wheel. larger combo diameters make the speedo read slower than the vehicle is traveling. I don't know exactly how calculate the change but if you search around you can find calculators that will tell you speed based on gear ratios and wheel tire combos

 

You are correct.
One example would be:
Stock GS-R tire ---> 195/55-VR15
195 is tread width in mm
55 is aspect ratio and of course
15 is nominal diameter of wheel
To find overall diameter in inches...
195mm * .55 * 2 = 214.5mm
Now convert mm to inches...
214.5 / 25.4 = 8.444" (close enough)
Add that to the wheel diameter...
8.444" + 15" = 23.444"

Assuming your speedometer is calibrated, 24" / 23.444" = 1.0237 (close enough).
With stock tires, your speedometer should read 100 mph while traveling at 100mph (duh!) ... and with the 24" tires shoud read 100 mph, while actually going 102.37mph.

There are several other "real world" factors involved in this, such as tire growth, etc., but I don't have enough knowledge to comment on this other than what I typed above is just a close "guesstimate".