Most modern cars have a fuel cutoff on zero throttle when revs are above a preset minimum (around 1500rpm).
Coasting down hill where the engine is ticking over and using fuel is less economical than lifting OFF the throttle and letting the engine use zero fuel. Then re-apply the throttle to maintain/re-establish your chosen speed.
This OFF/ON sequence can be just a few seconds and is overall more economical than coasting and more economical than maintaining a tiny throttle opening eg. on a flat road at 30 to 40mph.
If your variation in speed is of the order of 2mph (+-1mph on your chosen cruising speed) then it does not impinge on the speed of the following traffic.
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