If someone puts on larger auto tires, than the vehicle was originally
equipped with, will the Speedometer read faster or slower than the
actual speed?

For example, Lets say a car came with 15" tires and I put on 16" tires
(and rims). When the speedometer should read 30mph (with the original
15" tires), will it read about 25 or about 35mph with the 16" tires?

I cant seem to comprehend how to determine this.....