I have the same tire size in the rear. All of the math I have heard said that this should like 10% below the speed you are actually travelling. That being said, when I went to the track, the speedo was dead on when I crossed the line. Take that for what it is worth.
The answer is yes, changing your tire diameter / width / profile will affect the accuracy of your speedo. The effect is proportional to the change in tire circumference (Pi times the diameter).
Just in case your mathmatically challenged - here's a handy calculator to determine the affect of changing tire size: http://powerdog.com/tiresize.cgi
In your case, the change is small. In fact, I have the same size tires and part of the reason I went this way was to correct an over reporting speedo, i.e., it said I was going 80mph when I was really only going about 76-77mph. I am not aware if it is even possible to recalibrate the speedo.
97 tt BPU
P.S. How's Madtown? I went to school there in the '70's and would love to get back!
Yes tire size does effect your speedometer. Most people just match the tire diameter of what came on their car stock. So, if you were upgrading from 17s to 18s you would just need to put the proper lower profile tires on the 18in rim. What is the stock rear tire on the Supra?
Trap speeds at the track are an average between a point right before the finish line (66ft?) and the finish line. Your speedometer should be showing a faster speed than what is on your slip.