1995 speedometer reading fast

[95C1500]

I am having a similar issue, except it starts at around 3mph fast at lower speeds (~10 on the speedo, 6-7 real) and gets up to 6mph fast at higher speeds (70mph on speedo, 64 mph real) I have the stock tire size (245,75R16) and my speed sensors on the hubs match the GPS speed perfectly in torque. The only variance from stock is I have P rated tires instead of LT, but online the overall diameter is the same. Any ideas on what can be causing this?

Speedometer inconsistency is not linear. The higher the speed, the farther off it is.

 

[Mildphil]

Speedometer inconsistency is not linear. The higher the speed, the farther off it is.

What would be the cause of this?

 

[Michaelknight]

What should I do with my 97 MPFI I have 37s with 5.13s and I'm at 70mph when I'm really doing 50mph?