If all you are relying on is timing, you will never hit the end zone. Two motors given the same PWM value will not turn at the same speed, unless you are very, very lucky. It is a statistical certainty that two motors will not turn at the same speed for all possible PWM values, on all surfaces, at all times.

You really need a way to determine how much each wheel has caused the device to move, not just how far the wheel has turned. Even then, as you turn left and right, errors accumulate. For 10 foot runs with few turns, the error accumulation may not be significant. For a 100 foot run with dozens of turns, you could end up quite a ways off.