One thing to consider is that the car is not travelling faster because of bigger tires according to the speedometer. The car is actually moving faster, but it would not read faster. Less revolutions in the wheel to go the same speed. It would make for a faster top speed on radar but not on the speedo. As far as the speed/rpm relationship between cars, it is always going to be somewhat difference. Let's face it no two tachometers or speedometers are going to be dead accurate.
My 2 cents.