```
rpm = (speed in km/h) * (60 / tire circumference in meters)
```
Let's assume the tire circumference is 2 meters. Substituting the given values into the formula, we get:
```
rpm = (10 km/h) * (60 / 2 m)
rpm = 300 / 2
rpm = 150
```
Therefore, the rpm of the car tire at a speed of 10 km/h will be 150 rpm.