You state you are running 275/60/15s. They are 2" larger diameter than stock. That is why your speedo is off. There are speedo gears for the trans that would correct the accuracy but they are nearly impossible to find.
If you want to check your ratio, a simple test is jack the rear of the car off the ground, put it in neutral. Put a mark on the rear housing and yoke. Spin a tire by hand and see how many times the driveshaft spins for one rotation. That will also tell you if you have a posi or if it does and it's dead. Both tires should spin in the same direction.