Some observations: You can achieve better FE by improving Cd/CdA, and you can improve CdA by (to some extent) lowering the car, but for the same car at the same height, the lighter wheel-tire combo with better CRR will typically net better FE. Here I'm speaking about a car lowered via the suspension, since most OE VW wheel-tire combinations maintain similar overall diameters, so going to an 18in combination does not necessarily lower the car. Many swaps to 18in wheels will result in an increase in diameter, raising the car height, and countering (to a greater or lesser extent) the effect of lowering via suspension. Suspension itself can be a factor in FE, but the factors are complex enough I'll leave it out. Obviously production vehicles are tuned for the marketplace, but manufacturers do a lot of testing to improve the FE numbers on most vehicles (or for fleet averages). Not that the results are always accurate, but cars such as the e-Golf, Prius, or Insight are good indicators of what works to maximize FE, which is (one reason) why we don't see those cars excessively lowered, or with wider, low profile tires. The Cd is often improved though underbody panels, which mimics the FE found where some lowering might improve Cd , but allows for a taller and more narrow tire, which is key in these cars FE numbers.
Although the weight penalty of 18in wheels and the accompanying heavier tires hurts FE most in city driving, it still hurts FE on the highway, although all-highway trips may mask this, or may render the difference as negligible. If the 18in wheel-tire combo falls behind the 16in combo in any area of efficiency, the penalty of the 18's will grow. These factors go beyond offset and width (aero and resistance penalties), and include the Cd of the wheel itself, the FE loss or gain due to sidewall height, the FE loss or gain due to LRR tires, and FE losses or gains due due to the impact of height on RPM. It may also be more difficult to find better LRR tires in 18in which maintain the right diameter, and here we lose additional FE. Also, rolling resistance typically increases with speed rating, so if the 18in tires have a higher rating than 16's, they may also increase RRC, and lower FE.
I won't pretend I know the magic elixir, but from my experience, and tests here (going all the way back to Ernie Rogers) and elsewhere, the same car, under the same conditions, will see better FE with wheels that are more aero, weigh less, and have tires that are LRR, narrower, and lighter than the opposite factors. I initially also said "taller," but skipped it because I would then want to discuss other factors such as sidewall flex. That being said, there is also obviously diminishing returns, compromises, and the impact of one factor on another (such as taller tires raising ride height, increasing CdA, but lowering highway RPM, improving efficiency).
To the OP: Free bump.