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The effect you arre seeing is due to the displaced air being accellerated as it travels around the sides of the semi, so you are seeing a brief increase in your airspeed (a virtual headwind) as the semi passes you.
As the semi passes, you first get a surf draft (for better mileage) as the air displaced by the bow shock off the front of the semi helps to fill in the low pressure zone trailing your vehicle, followed by a period of increased airspeed (for worse mileage) as the side of the semi passes you, followed by a side draft (for better mileage) as the semi pulls just ahead of you and its trailing low pressure zone helps to pull off some of the air being displaced by the leading edge of your vehicle.
In my aerocivic, the surf draft has no effect since with the boattail I don't have a low pressuse zone trailing my car, at normal highway speeds the accellerated airflow along the sides has little noticable effect on my car, and the side draft has only a very minimal effect on my mileage.
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