Comments rsiddall has made

  • Or maybe slower

    I've been thinking about this and I think some vehicles will use less fuel if run at the minimum speed to pull a lower gear than top gear.  Take a car that turns over at 1500 RPM at 30 MPH in one gear and 40 MPH in top gear.  Even though the engine fires more times to do the journey at 30 MPH, because the drag force is much lower the carb or fuel injection may meter so much less fuel into the engine that the net fuel use is less than if you did the journey at 40 MPH.
    I remember being told (in the 1970's, I think) that the drag force was approximately equal to the engine loss at about 35 MPH.  Presumably now we run thinner oil, the engine loss is less in modern vehicles, so drag starts to predominate at lower speeds than that.On Umbra on speed limits posted 4 years, 2 months ago 10 Responses

  • Almost right

    Sorry, the laws of physics have not changed since the 1970's.  The most efficient speed of a vehicle is still around 30 to 40 MPH.  The drag coefficient of a new car may be a bit less than one from the 1970's but that doesn't affect the most efficient speed, only the overall fuel consumption.
    The most efficient speed will be the minimum it can pull the highest gear comfortably.  If you go slower you'll have to use a lower gear and the the engine will fire more often to go the same distance.  If you go faster, your drag losses will increase and so will the engine losses as the engine turns faster.  This increases the load on the engine and the carb or fuel injection meters more fuel into the engine each time it fires.
    The laws of physics aren't any different for SUVs. They use more fuel as they're larger, and so have a bigger frontal area to multiply the drag coefficient by.  They're also heavier, so they take more energy to accelerate and decelerate.On Umbra on speed limits posted 4 years, 2 months ago 10 Responses