I have a similar situation here, a 75 mph speed limit two lane highway. I normally drive 65 in the slow lane, but I can tell it definitely causes some congestion and safety concerns. Normal traffic on this highway does 80mph and the constant stream of 18 wheel trucks does 75mph. When the truckers come up on me they have to brake hard if they can't switch lanes right away, and then pass causing a backup in the fast lane. And this is if I am doing 65, any slower and real safety concerns and near rear end crashes happen, I gave a go for a short while at 55mph cruising where my car would get 80 mpg, but it was way too dangerous, and caused many problems on the interstate for other vehicles.
You have to consider how much gas you are costing everyone else who has to slow down and re accelerate to pass you too. Is it worth that few mpg to make a semi truck use a half gallon of diesel to pass you?
10mph under the limit would be the absolute limit. And if you even think you might end up annoying someone, try not to. A few more MPGs isn't worth inspiring the ire of others. But if those annoyed people are riding in your car. fek 'em.