The dealer is correct, there are no adjustments for the
speedometer/odometer.

I've never measured tire circumferences, but there are probably variations
in tire circumference between tire makers.

Tire size alone does not explain the differences you are experiencing.  The
shorter distances mean that the new tire's diameter is larger so it turns
fewer times for a given distance.  Tire diameter will vary about 1/2 inch
over the tire's life due to tread wear.  Theoretically, a larger diameter
tire should improve fuel economy.

Different tires will have different effects on fuel economy, as will
different inflation pressures.  For improved fuel economy and tire life, I
recommend inflating the tires about 4 or 5 PSI higher than the
manufacturer's recommend tire inflation pressures, although this will result
in a slightly harsher ride.

New tires don't mean you need to recalibrate anything. Even if the tires
are a different size, the difference in the speedometer readings will be
very minimal (< 1mph). It certainly should not be affecting your MPG
that drastically. When you state your MPG, are you calculating this
based on actual calculations or are you using the car's MPG display?
Perhaps your car's MPG calculator was reset when you had the tires
changed. In this case, it will take some driving before the computer
gains enough inputs to bring itself back to the average you were
getting before.

Make sure that the new tires were balanced properly after they were
mounted. Improper balancing CAN cause a drastic change in MPG as well
as nasty vibrations in the steering wheel at highway speeds.