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Frank is right, motorcycles have cams that are ground to open the valves wider and to help the engine rev higher, higher revs tend to give poor mileage because of slow flame spread, this type of cam grind also requires a lot of motorcycle engines to idle at a higher speed.
There are a few motorcycle engines that are exceptions to this, I think Honda Goldwings are one of them as they tend to turn rather slow because a 1,800cc engine on a motorcycle isn't working very hard to move it down the road, then of course there have been a few motorcycles like a version of the Honda CB400 that was sold mostly in Japan that had a V-tec engine and was getting pretty decent mileage but still had a high power out put at high revs (of course using a lot more fuel at that time).
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