You decided to go with the 6" lift. Are you going with the BDS coilovers? Show us pictures when she's done.
The lift won't have any thing to do with the speedo reading the tires will.
With 34" tires you'll be about 1mph over on every 10mph examples:
Speedo 30mph; actual 33mph
Speedo 50mph; actual 55mph
Speedo 70mph; actual 77mph
Speedo 90mph; actual 99mph
There is a pattern there.
You don't have to recalibrate it. You can if you really want to. I just go the speed limit and get there a little faster.
I usually go 5mph over most of the time anyways. Since I have 33" tires I just do a couple mph over or the speed limit depending on how fast I'm going.
I know a few police officers and they say they don't start looking at anything until it's 7mph over. 8 and it starts to gets their attention. Unless you're in a school zone. Don't mess with our kids safety.
What two things you should think about is
1. figuring out how to reset the yaw sensor, it determines the unloaded posture of the truck, it needs to be reset since the center of gravity is higher than stock and
2. the PCM programmed for the bigger tires otherwise this will throw off the stability control and can cause issues.
Those are the 2 things I'd like to know how to do on my 2017 TRD OR after my 3" lift and 33" tires.