Let's not use confusing terms that could be interpreted differently.
Speedo runs off the gearbox output shaft, so it takes its reading based solely on engine rpm and the gear selected, irrespective of what's happening at the tyres and diff. Therefore, 2000rpm in 5th will always make the speedo read the same speed (say ~80km/h) no matter what size tyres or diff ratio you have.
Increase your tyre diameter and the vehicle physically goes faster but the speedo reading stays the same. So if your speedo reads high, putting on larger tyres will make it more accurate. If it reads low, putting on larger tyres will make it more inaccurate.