This modified chart from a "Aircraft Powerplant Maintenance" book published
by Avotek illustrates why a geared rotary engine is easier to design
a carburetor for. I have added typical mixture ratios on the right
and a 2.85:1 geared rotary engine RPM scale on the top. Also a few colored
lines.

A street car is very much concerned with the range from the green line
to the 600 RPM idle speed. The vast majority of circuits in a automotive engine
and direct drive aircraft engine deal with this RPM range. The rotary won't
idle at all with a typical gear box at less than about 1600 RPM. Typically we
allow them to idle at 2000 RPM which gives a prop RPM of 700 with a 2.85:1
gear box. The rotary is pumping more air at idle so there is higher velocity
through the venturi. This produces a better mixture signal.

The range from the blue line to the red line is the geared rotary
aircraft range. In fact the cruise range is even smaller as it varies from
about 3500 RPM to 6000 RPM. Building a carb to deal with this restricted
range is much easier than building an automotive carb or even a direct drive
aircraft engine carb.