The above link also includes a link to a
SAE study of a full size SUV and a full size sedan comparing windows down vs. windows up+A/C on a range of speeds: 50, 80 & 110 km/h.
In the on-road test, they noted that (a) windows down can have a measurable effect on fuel consumption, and (b) the MPG hit was higher for both vehicles at the 3 speeds tested when the A/C was run with windows up, vs. A/C off and windows down.