It has been suggested here that at cars limits of adhesion at 100 mph
are so different from the common experience at 75 mph that one must
explore them fully at 100 mph if one is to be "competent" at that speed.

Let's look at some numbers:

A Porsche 996 (the official designation of the current 911) generates
136 kg of rear lift and 64 kg of front lift at 157 mph. The cars mass is
about 1400 kg so that is a reduction in weight on the wheels of
approximately 1/7.

<http://www.pacoza.co.za/spec-996.htm>
<http://www.autozine.org/technical_school/aero/tech_aero.htm>

But! Aerodynamics gave a square law that comes into play as well:

The lift of a body is proportional to the square of its speed. You can
understand this from the fact that the amount of air encountered is
proportional to the speed and the change in velocity of that air
encountered is *also* proportional to that speed. Twice as much air,
being deflected twice as fast results in four times as much force.

Hence at 78.5 mph, a speed at which I'm sure most people would agree
most drivers have a lot of experience, the lift forces are going to be
roughly 34 kg at the rear and only 16 kg at the front, for a grand total
reduction of only 1/28 of the cars weight; about 3.5%! And at 100 mph,
the numbers are going to be 55 kg rear and 26 kg front, or a reduction
of about 6% of the vehicles weight.

So in going from 78.5 to 100 mph, the driver will experience a reduction
in weight (and thus limit of adhesion) from 96.5% to 94% of the vehicles
theoretical maximum. Not an outlandish change with which to deal, I'm
sure you'd agree.

But there's more! The Porsche 996 has a body shape particularly suited
to generating lift in the first place and thus has a spoiler to deal
with that lift (which in particular lightens the back end of the car and
thus changes the handling balance), and those numbers are for a 996 with
its rear spoiler *down*. Would not an examination of the numbers with
the spoiler *up* perhaps be more consistent with what we might find in a
road car of less extreme shape?

Spoiler up:

Speed         Rear       Front    adhesion available
157 mph       14 kg      5 kg           98.6%
100 mph       5.7 kg     2 kg           99.4%
78.5 mph      3.5 kg     1.3 kg         99.7%
60 mph        2 kg       0.7 kg         ...

I trust everyone can see how silly this is getting. To suggest that a
typical road car's limits are going to change radically between 60 mph
and 100 mph is just ludicrous.