Measuring time the vehicle takes to accelerate in iPhone

Question

How to measure time the vehicle takes to accelerate from 0 to 100 km/h (0-60 mph) in 400meters (a quarter mile) using iPhone accelerometer?

I have situation where i

Answer 1

the accelerometer can only be used to measure G-forces.. not speeds.. You will need GPS to measure when you've reached 100km/u speeds..