What is the right clock chip for making a speedometer with an AVR
Your speedometer needs to measure the elapsed time between two pulses, and it doesn't need to know the time of day.

I'd consider using the AVR's calibrated internal oscillator. Its +-1% deviation from the nominal frequency is probably more precise than the circumference of your vehicle's tires.

If that's not good enough for you, spend an extra fifty cents for a crystal oscillator.