The ECM/TCM module handles the pulses from the speed sensor and calculates the car's moving speed.. The programing in this module is encripted and the only one able to access this program, the data, and the lookup tables (so far) is KIA.. Being off by 4mph "could" be corrected by going to a different size tire.
Your 195/60/15 could be bumped up to 195/70/15
at 70mph their calculation says the speedo would read 74.44mph
(an increase in shown speed of 4.5mph)
Add that (4.5mph) to your "slower" reading of 65mph when you are doing 70
and the speedo should read 70.5 when actually going 70.
Not too far off from actual... Without any extra mods...
See this calculator to see what tire size can do..
Tire Dimensions Made Simple - Discount Tire