The speedometer is electronic.
Why would you put smaller tires on your car?
The difference in speed reading might be negligible, calculate your speed by timing the time from one mile marker to the next on the highway. Obviously, do this on a straight, level road, with cruise control on, and the speedometer reading a mutliple of 5, ie 30, 40, 55 mph. Then match your calculated speed to the reading on the speedometer.
Have fun!
