[Shotimes] towing and fuel economy

[van Oss]

Today I used the Crown Vic to haul away two trailer loads.  While driving I 
got to wondering about something.

When I'm towing a load, part of the load is mass (affecting acceleration and 
braking), some is friction (trailer tires and axles), and some is air 
resistence.  Overall, fuel economy is lower than when the car operates with 
a trailer.  Okay.

But could someone please explain exactly how that plays out in an EFI 
engine?  Let's say that the car, versus car and trailer, are driving down a 
level road at, oh, 25 MPH.  At a given speed in a given gear, the engine 
revs should be the same, yes?  Or am I wrong?

So granting that the engine has to "work harder" under tow, exactly what is 
it doing differently?  Does the computer change the air/fuel ratio? What's 
going on in there?!?!?!

TIA for the education.

VO