Looks like a 6.5-square-inch reduction in frontal area.
How it would affect the actual Cd we couldn't say with certainty.
For the 1982 Pontiac Trans Am Firebird, changing between 205mm and 195mm meant Cd 0.284, a delta- Cd 0.005 ( 1.5% )
185mm = Cd 0.278
175mm = Cd 0.273
165mm = Cd 0.268
One might plot the knowns on graph paper, create a trend line, and then extrapolate down to narrower tire estimated Cds.
Exploding tires are over-rated for fun, so be mindful of your load ratings.
