To calculate the percent uncertainty, you can use the following formula:
\[ \text{Percent Uncertainty} = \left(\frac{\text{Uncertainty}}{\text{Measured Value}}\right) \times 100% \]
In this case, the measured value is 312.0 mph and the uncertainty is 3.9 mph.
Substituting the values into the formula:
\[ \text{Percent Uncertainty} = \left(\frac{3.9}{312.0}\right) \times 100% \]
Calculating that gives:
\[ \text{Percent Uncertainty} = \left(0.0125\right) \times 100% \approx 1.25% \]
Rounding to three significant figures, the percent uncertainty is approximately 1.3%.
Thus, the correct response is 1.3%.