tire size affecting speedo reading?

[sml]

Hi, I was wondering which way was it that tire size affect speedo reading. Is it if the new tire OD is bigger than stock tire OD, then your actually going faster than what your speedo says? Cause speed is a function of distance/time, so your wheel hub rotates the same amount, but if your tire has a larger diameter, then that means you've travelled farther than it's suppose to, so you're acutally going faster than what the speedo reads.
I'm asking because I was in a friend's car and they got a speeding ticket and we didn't think we were driving that fast! The thing is that they just put new winter tires on and it's one size smaller (15" vs. 16") I'm not sure if the new tire OD matches the new tire OD, so I think it might have something to do with it... Was there a thread where someone got off a speeding ticket after finding a speedo error from a speedo test? Where can you get a speedo check and how much? Do you think they should go to Pointts to try to fight it off? THX!

[Blue Devil 2]

Ok goto Mr. Spedometer...it will cost about $50.00 , and If your wheel is bigger than your stock wheel than your actual speed will be slower than what your spedometer says.

There was a previous thread with good suggestions, but goto points, then they will tell you to goto mr. spedometer, then you will be able to fight the ticket no problem.

[sml]

Actually, just found this site:
http://www.miata.net/garage/tirecalc.html
Increasing wheel diameter increases actual speed in comparison to stock speedometer reading...

[drox]

So basically when you go to get a speedo test done, bring some 20in rims