On 98 and newer trucks, there isn't a speedo-gear in the t-case, like there was on older trucks.
Instead, they use the RWAL tone-ring attached to the ring-gear in the rear-differential.
So, as you change tire-size, the rpm of the axle changes vs. the speed you are going, which changes the speedo reading.
If you put in steeper gears, the tone-ring still spins at the same rpm it did before, so long as you haven't changed the size of the tires, the gears are "upstream" of the tone-ring, so they don't have an effect.
In the older trucks, the speedo/odo ran off the rpm of the rear driveshaft. So as you increased the size of your gears, the speedo goes up. So if you went with 10% taller tires, and 10% taller gears, the speedo would be about as accurate as from the factory (i.e., not).
But, with the new trucks, 10% taller tires + 10% taller gears means you're going 10% faster than the speedo says.