I use Tone's scan gauge and its speedometer readings after putting on 285' Nittos. There is quite a difference in speed, something like ten miles more actual at 80mph. So if I am reading 80 on the speedometer, the scan gauge will read that I am actually going 90mph. The good news is that this also means that I am getting better mileage than I read on my odometer. Of course, the scangauge calculates everything for me, trip, mpg etc.....
One thing you will notice is that you loose torque, hence your acceleration will be reduced. Off road I love my nittos. Noise wise I have not noticed a difference. Handling in curves feels better to me than with stock tires.
From Unimog to VX - a serious change in velocity - a small sacrifice in voracity.