On Wednesday, July 19, 2017 at 2:39:02 PM UTC-4, Mad Roger wrote:
On Wed, 19 Jul 2017 11:17:25 -0700,
Bob F wrote:

Only if you try to maintain the same acceleration level.

At cruising speed, say 60mph, the main component of force is the drag
coefficient, isn't it?

So you need torque at the wheels sufficient to overcome the wind
resistance, even if you maintain a set speed of 60mph.

If the needed torque to maintain that speed of 60mph is 100 Newtons
(plucking a number out of thin air), and your engine has to work harder to
generate a torque of 100 Newtons, then we may not get the fuel economy that
we thought we'd get.


And there is where your physics goes wrong. Torque is not measured
in Newtons, it's in Newton Meters. You're completely neglecting the
distance part. Sure, the engine applies less torque to the wheels
with a smaller diameter tire, but it also has to apply the force
through a longer distance. With a large tire, it's more force
but through a smaller distance around the tire. With a small tire,
it's less force but you have to go more than the one revolution you
did with the large tire to move the car the same distance. The
*energy"* is the same.

Now there could be other difference between those tires that affect
the rolling resistance, etc, but the basic torque physics are as
above.

What you're arguing is like saying that the energy required to move
a bicycle up a hill is different, that you can do it with less
energy input, based on the gear ratio. There is no free lunch.