AO Silver Member
Join Date: Nov 2003
Location: United Kingdom
County: Perth and Kinross
Re: What speed is most fuel efficient?
As has been said already, about 40 mph is where the best fuel economy is achieved in 5th gear. Below that speed, and although the effects of aerodynamics are close to negligible, the engine runs very inefficiently. As it is so far off maximum torque, efficiency suffers due to poor volumetric and thermal efficiency. The variable valve timing helps to minimise this reduction in efficiency by increasing cylinder filling at low engine speeds.
Faster than about 40mph, the greater throttle opening does allow the engine to run more efficiently, but aerodynamic drag also increases which requires more fuel to make progress.
[SIGPIC][/SIGPIC]Drive sideways- life's too short not to
Slip-sliding my way through winter. On four wheels and two.