I know this has been brought up many times so far, but as time goes on, it's becoming increasingly common for auto-makers to eliminate V6 and V8 engine options with turbocharged I4 and V6s. This is currently the case on the Optima / Sonata twins, the Kia Sportage, the Chevy Sonic and Cruze, the Ford Flex and the Buick Regal to name a few, but the next generation Fusion and Malibu are adopting that model, the Dart will have 3 I4 engine options, Hyundai is releasing a turbo version of their Veloster, etc... From what limited review I can manage based on people nice enough to include turbo or turbo only trim levels in their vehicle description - this can make up to a 30% difference in fuel economy.
I understand that you're trying to make this site accessible to people who don't know that their hood opens, but you've seen fit to add hybrid and diesel options to the drive-train, and turbo motors seem to have as significant an effect on fuel economy as electric assist. Yes, there are sites that tabulate this data - like the EPA - but they have very few data points and don't seem to be setup to track data as well. Ultimately, it's a one time inconvenience for the user to register their car, but it makes your data considerably more useful.
Thanks for the suggestion. As you noted we're not tracking turbo at this time and we don't currently plan to. (Here's the FAQ on this: Why don't you track more vehicle details?) But we're constantly thinking about these things and we appreciate the feedback. As always folks are welcome to include this information in the vehicle name or description—it's a big help to others who want to see more details.