Well... Guess thats fair enough... The original tire size as far as i know is 184/60-r15 (it was on my corolla)...
Im from Europe where we count in km/h (Kilometers per hour) instead of mph...
So to do the calculations a bit easier for me ill start by calculating the numbers in to km/h!
66mph = 106kmh (give and take a kmh)
70mph = 112kmh (give and take a kmh)
When changing from the standart wheelsize to your wheelsize there will be a speedometer variation of +3,4km/h (+2,1mph)...
And from factory a +/-10kmh (+/-6,2mph) variation from speedometer to actual speed is allowed..
So as far as i can see your speedometer is only about 1,9mph(3kmh) off...
Thats not a bad result... here in Denmark where i live the Police force take the +/-10kmh in account when trying to catch speed sinners...
So if they get you running 120kmh where only 110kmh is allowed they let you go... Even though they use laser, radat and other precise instruments...
And thats just because of the error margain from the car factories
Hope this could help you a little
:Edit:
Sorry... Guess you might want to know why the speedometer dont match you actuall speed... Its because of the gearing of the car change when you change the size of your wheels... And the speedometer is made for a very strict gearing... You cant change the settings of the speedometer as you can change you gearing...
Therefor you will find these small error displays...
But they dont really matter much...
If you one day see a diffrence from your and your dads car about 15mph... Then i guess i would get my dads car checked if i was you (you own a Toyota... They are always reliable
)