Under Iowa Code section 321.278, the state's drag racing statute, traveling in excess of the posted speed limit is an important element of the offense. Testimony along the lines you describe may generate reasonable doubt as to guilt. Officer may attributing sound of Cobalt to the Mustang, especially if he didn't visually observe cars exceeding speed limit.
Consult a local Traffic Ticket Attorney - usually inexpensive & free initial consultation. You may have a good defense to this citation
This is not intended to be legal advise or as legal representation. I am a California personal injury attorney . Be aware that every state has its own statute of limitations; and statutes & case laws that govern the handling of these matters.