It depends on the area.
Out in the country near the Missouri state line the county sheriff drives a 4wd Dakota.
Down in Little Rock, though, they use repossessed tuner cars to infiltrate the street races.
There's a city in NWA taht uses a mid 90's Trans Am as a pursuit car.
And there's a city in Central Arkansas that has a Challenger pursuit car.
They're kinda behind the times though because a lot of the precincts still bend the block in 9C1 Caprice's.