Well i know i get better gas mileage when my turbo is creating boost, so i actually 70-80mph i get my best gas mileage. in my sisters N/A no faster than the speedlimit in that thing....speedo tops at 85mph but the engine can go further...or so i'm told :wink:
Mine's an N/A and I can manage just shy of 30mpg if I set the cruise at 70. Not sure what I would get if I set it to 55...never done it. . But I'd say that 29-30 is pretty good by today's standards. Someday I'll have the money to buy my hybrid.
The thing about setting interstate speed limits at 55 is that I think there's a good chance it would be counterproductive. A universal 55mph speed limit was used in a day when there were fewer cars on the road, and so our highways did an OK job at handling the traffic volume. Today there are more cars on the road, and even with a 65mph limit there can be problems on high-volume days. I know most of us have had the exprience of sitting in traffic not because of an accident or bad weather, but because there were too many people for the road. If you slow everyone down, traffic only piles up more quickly because each car is on the road for a longer period of time. The result: more interstate parking lots.
The force of drag may go up with the square of your velocity, but the lawmakers need to consider that you get ZERO MPG when you're stopped in traffic idling the engine.