With bigger wheels you will go farther per RPM making the car go faster.
The distance a car travels with one turn of it's wheels is equal to the circumference of one wheel---assuming those wheels have the same diameter.
Put larger wheels on the car and the circumference increases and therefore so does the distance traveled.
And since the speedometer is calibrated to the wheel diameter that came with the car, if you increase the wheel size, you will be speeding as measured by a radar gun, but your speedometer will tell you that you are only going the speed limit because it will not be properly calibrated.