http://www.metrompg.com/posts/bike-trailer.htm

As strange as it may sound at first, my car's average fuel economy has
declined because of a small cargo trailer I built this summer to pull
behind my beater 10-speed bicycle.

That's because driving techniques typically permit me to get higher
fuel economy in sub/ex/urban driving than I get on longer (higher
speed) highway trips. So every time I use my bicycle instead of the
car for a local trip, the proportion of highway driving in a given
tank of gas goes up, and my overall fuel economy goes down!

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