Those were not out for fuel economy testing, all of that testing is done on dyno rolls with precise measurements of the emission(even in fe tests) I ran hundreds of those tests when I worked in development. They could have been out getting street miles for a variety of final checks and/ or troubleshooting, could be as simple as chasing a squeak. They may simply have been out for evaluation. Some engineers have favorite roads to evaluate, looking for whatever they are looking for. There are replicas of certain roads on the grounds for the ride guys. Sometimes we have drivers just out driving them to log miles. There's been a bunch of new GM pickups cruising Mound, saw some out there last night.
I see Chrysler vehicles out in groups all the time around Auburn Hills, especially on Telegraph, heck I saw some Dodge trucks with yes trailers in 75/Baldwin area this afternoon. One had a ballasted trailer with a 8'x5' flat panel to simulate the front of an enclosed trailer. See stuff all the time.
C5 base tires were 245 and 275 front rear, and those were Eagle F1's iirc. Not incredible tires, certainly by today's standards.
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