How is speed read?
 

So im taking the jump to increase the tire width on the stock rims..

Front 265-35-19
Rear 305-30-19

I know there are speed calculators out there, but since both tires are being changed? How would that affect what the speedo is reading? Is speed being read by rear or front tires? If there is already a post about this, please point me in the right direction...

Speed indicator usually comes from the transmission and the rear tire size changes the speed of the transmission output to the speedometer.

So speed is only read from rear wheels?

Here is one for speed:

Tire size calculator

Here is one to ensure that you have the proper staggered sizing:

Tire Size Calculator - tire & wheel plus sizing

basically dont worry about it. the factory speedo is slow by 2-3 mph anyways.