Tire & wheel weight will definitely have a slight effect and, as some have pointed out, each pound on the wheels counts more than, say, a pound in the trunk (or in the driver's seat). And the more accelerating and braking you do the more it matters. Don't forget also that increasing the unsprung mass leads to additional losses because the tires must absorb stronger hits when going over bumps. Also, if the tires & wheels are heavier because they're fatter, then the aerodynamics will suffer, too. And cheapo steel wheels with stock hubcaps probably have better aerodynamics than most cool-looking alloys.
A little theory:
The linear inertial energy of a car (or its wheels&tires) is
Elinear = (1/2)*m*v^2 ,
where m is the mass and v is the vehicle speed. That means you have to feed in at least that much energy (from the engine) to accelerate the car up to that speed.
Additionally, the rotational energy of a wheel/tire is
E rotational = (1/2)*I*omega^2 ,
where I is the rotational moment of inertia (see
List of moments of inertia - Wikipedia, the free encyclopedia) and depends on the mass distribution.
In the worst case all the mass is concentrated at the outer radius (a hoop); in this case I = m*r^2 where r is the radius.
An intermediate case is uniform distribution (a disk, probably a bit optimistic); I = (1/2)*m*r^2.
The ideal best case would be a point mass concentrated at the axle (I = 0), but you can't build a wheel like that (except out of unobtainium).
The rotational speed of the wheel in our case is omega = v/r, so for the hoop we have Erotational = (1/2)*m*v^2 and for the disk it's Erotational = (1/4)*m*v^2.
So comparing to the linear energy you can see that each pound in the wheels & tires indeed counts 1.5 to 2 times.
Some practice: Every year when I switch from summer to winter tires (both 185/60R14 from Continental, same circumference) I notice that the car accelerates a bit differently in 3rd gear. With the summer setup (alloy wheels, weigh 1 kg (2.2 lb) less each; i.e. 15 kg instead of 16 kg for wheel&tire) the car has a surge of acceleration in a certain range of engine speed, whereas with the winter wheels&tires (steel rims) the acceleration is more constant. So even the 6 to 8 kg of equivalent additional mass can be felt on the butt dyno. The perceived difference is more pronounced even than adding a passenger. Go figure!
What I also immediately notice is that my winter tires actually have *less* rolling resistance than the summer ones, even though I'm running the winters at lower pressure. I suspect it's because they're a newer model, where they paid more attention to fuel economy when designing them.
BTW on an earlier thread others found the opposite
http://ecomodder.com/forum/showthrea...ires-4838.html , but remember that in the winter other factors (longer warm-up time, less-potent winter gasoline, more air drag at a given speed in colder air) will also take their toll on fuel economy.