The only geared bike that will make you less tired is an ultra light weight one with the proper cassette and gearing choices which would run you into the thousands.

The only explanation I can offer you as to why your SS is making you less tired is there is less mass in the rear and overall drivetrain, which would only have an effect based on what kind of riding style you have.

Personally I like riding both, and either type of bike makes me tired in different ways - with the road bike I push harder, the single speed gives me a great workout overall.