Will 195 60 15 size tires affect GPS positioning ?

Nope! GPS works off a signal from a satellite. It may, however, affect your speedometer depending on the difference in circumference from the OEM tire.

 

There is a menu somewhere in the NAV system (look for it - I don't know off hand where it is) that lets you put it into calibrate mode when you get new tires.

 

Man, Toyota engineers think of everything! Does this calibration make adjustments to the speedometer as well, or is it just to make the GPS arrival time calculations a bit more accurate?

 

The OEM Nav system includes a dead reckoning system, which helps the Nav system keep track of your location during loss of GPS signals. Setting the calibration on the Nav system improves the calculations, but does not affect the speedometer.

Tom

 

Factory GPS systems often use the odometer and a compass or accelerometers to update location when you don't have an adequate GPS signal.

The one in my Chevy does a reasonable job of keeping track of position and direction in large parking garages and long tunnels without GPS signal. It's made by the same company as the Toyota system.