Wheels have thrown off Speedo
 

I've found out that my speedo is now reading 2mph slower than my actual speed. I believe this is due to my changing wheel sizes from 195/50 R16 to 215/40 R17. I know if you go based on the numbers there isn't a big difference, however I talked to a wheel/tire guy and he said that those numbers are exact. a 195 might actually be 190 or 200 or a 215 might be 210 or 220. before you ask. I checked my speed with a higher end portabel Garmin GPS systems a friend of mine has. And I'd prefer that my speedo be right on the money or stating a slightly faster speed than I'm actually traveling.

you must remember that your speedo wasn't EXACT before you changed....and believe it or not....the tire size you went to is the closest thing made to the stock tire diameter in a wider footprint...

I don't know if you can have your speedo RECALIBRATED, but if so...that is the ONLY WAY you are going to get it exact.