Bud, the size of tyre will not affect the speedometer, only the actual physical speed. The Speedometer gets its readings from the gearbox sensor, of which is a gear reading how many times its spins multiplied by the selected driving gear, thats whats outputted to the end of the axles and to the wheels.. what the wheels do with it then is out of the control of the speedometer.
I have a 5spd auto in my mpv also, and I only wish my rpms were that low on the highway in the 6.
Also, keep in mind that the stock tach is inaccurate on the 6, to the tune of almost 200 rpm on my car when near 4k rpm. I have a scan tool connected to my obd2 that has an rpm readout and even at idle it's almost 100 rpm off.
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