Speedo swaps on singles?

[DreyeIce]

On my bike, i have the drive gears on the front wheel (like alot of other singles) but anyways, i had a 0-80 A.Rolle speedo the there, but im getting a NOS 0-100 a.rolle speedo to replace it,

my concern is will it read the same? like will 20mph be 20mph on both speedos? are they geared internally the same? one just goes up to 100?

thanks,