Depends on the car but it should be between 0-5% off depending on the speedoeter, assuming you're using the correct size OEM tires. The 5% off is for the high end and it's always going to be 5% short not 5% long, as in instead of 1 mile, you're car's only traveled 0.95 miles. For example my Mazda was about 1% off, My Toyota was about 4% off and my Mercedes is spot on as verified by GPS.
Now if you're using a motorcycle, all bets are off, motorcycles are usually 7-11% off. My Kawasaki Ninja was consistently 8% optimistic about how fast/far I was going/gone.