Is it a fuel-efficieny car or a fuel efficient car?

In school, I learned to combine two words that modified a third. As such, I've been writing "a fuel-efficient vehicle" rather than "a fuel efficient vehicle". This would only apply if the third word was a noun though, so I would write "personal transport-related" rather than "personal-transport related".

Now, I'm writing a peice on, you guessed it: fuel efficiency. I've been doing some research and I'm noticing that no one anywhere is combining words like I do. I write "fuel-economy standard", the write "fuel economy standard". I write "fuel-efficiency increments", they write "fuel efficiency increments".

Where did I go wrong?