The 2014 Acura RDX only comes with one engine: a 3.5-liter V-6. That doesn't sound like the recipe for segment-leading fuel economy, and it isn't. But for its power and purpose, the RDX's fuel economy ratings aren't bad.
The front-wheel drive model achieves 20 mpg city, 28 mpg highway and 23 mpg combined, while the all-wheel drive model is estimated at 19 mpg city, 27 mpg highway, and 22 mpg combined. That's about on par with other luxury crossovers--although not quite as good as the BMW X3 with its turbo four, for instance.
Beyond that, you don't have any other option for the RDX; as of yet, at least, there's no hybrid, and the diesel engine that Acura had been teasing a few years ago (which would have been perfect for this model) never arrived.