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Originally Posted by bull27
I read somewhere on this forum that 4300-4500RPM range is the most fuel efficient, I might be completely off on this? In a 5 speed Boxster that would be 4th gear on the highway (approx 70-75MPH). I dont know about you guys but in Massachusetts there is no "cruising" at 80MPH, that is a ticket for sure, especially on the Mass PIKE!!!!!!
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The tallest gear ratio available operated at the lowest RPM's, without lugging the engine, will provide the best fuel efficiency. Smooth driving is key. Limit acceleration and abrupt lane changes.
Also, remember that drag increases in proportion to the square of speed. This essentially means you experience more than twice as much drag at 80 mph than at 55 mph - even though you're only going 25 mph faster.
So, cruising at 80 mph instead of 55 mph can result in as much as a 25% decrease in fuel efficiency. Obviously, this is completely dependent upon the drag coefficient of the vehicle. It's entirely possible that a Boxster is more efficient at slightly higher speeds than less aerodynamic cars. But, in general, a vehicle begins to lose fuel efficiency above 55-60 mph.
It's difficult to say what kind of penalty you'd have with the top down. That would be dependant upon the car's drag coefficient up versus down. But, my experience is that the top down significantly alters the aerodynamics of the car - and not in a good way. So, I would also expect to see a corresponding decrease in fuel efficiency.