While this isn't really news to most of you I thought it relevant for newbies.
Over past 6 years I have been swapping back and forth between 15" and 17" wheels. I've had a variety of tires on each wheel yet no matter which tire I, choose I observe an instant 4-6mpg hit when I put the 17s on. Lately I have observed an even larger hit because it is easier to employ hypermiling techniques with the 15s because they roll so much easier.
My current setup
15x7" oem alloy wheels (14-15lbs) with Yokohama dB Super E-Spec 185/65/15 tires
17x7" Centerline RPM wheels (15lbs) with Michelin Primacy MXM4 215/45/17 tires
The 17" tires weigh more so the total difference in weight is about 6 lbs more per wheel.
Both tires are rated as low rolling resistance but the extra width of the 17s and possibly tire compound/construction makes them not roll as easily as the 15" setup. Using the same hypermiling techniques in the same parts of my commute will net 50mpg with the 17s and 57mpg-60mpg with the 15s. It is important to note that my commute consists of 98% freeway driving at 60mph with an elevation gain of 1,200 ft. and a total of 96 miles.
In summary the smaller and lighter 15" tires make a pretty huge difference when compared to the wider and heavier 17" tires despite them being LRR. Now consider what this means for high mpg cars that come factory equipped with 17" or 18" tires like the Lexus CT200h or the 2012 Camry hybrid which are both EPA rated at approx 42mpg combined and are rated with the 17" tires. If one were to swap out the 17s for 15s their mpg could potentially rise to 50+mpg! Thankfully I have a friend with a CT200h who is going to perform the swap to see if this idea pans out.