Come on fellas (and I can say this in EK as we are all sitting on a pair of pills (hopefully)) if ATC give a speed constraint that is unacceptable for whatever reason tell them what speed you CAN maintain. Accept the extra track miles and still save gas. The ATCOs I know are good buggers but dont have an intimate knowledge of limits on our airplanes. p.s. assigned speeds are +- 10 kts so forget about pulling selected speed if managed gives you that range.