This can really vary by both state and local laws, plus department policies. I would take an educated guess that when marketing police vehicles, most car makers will be a bit flexible to adjust to local standards. I don't believe there is a national standard though.
In Florida, the only standard I'm aware of in state law is the paint that goes on them. Sheriff's office's can only have certain colors, PD's can only have certain colors, and FHP is only allowed Black and Tan on their marked units. However, each department may have their own requirements as to equipment, engine specs, etc. Most of the time, COST drives most of those decisions.
"I would rather my boss give me a butt kicking for being over the top than a eulogy for not being thorough!" ~~~~~ Aussie George
"It's an American police station. Guns are easier to find than a working stapler." ~~~~~ smcc366