Big wheels causing problems

On my CRX, the stock wheels were 157/70r13. I now have 205/40r17 80H. I don't know exactly what the 80H means but they are Kumho's. I think that my speedo is off by 5 mph. I got pulled over for doing a little under 30 in a 20 zone and my speedo said 25. How can I figure out exacty how much it's off by. The circumfrence of the stock ones are 70 inches, the after markets are 75. Does this help anyone help me?

try getting 205-35R17.. that may help

try this out

http://www.miata.net/garage/tirecalc.html