To start off, the C6 turned out to be a very fine long-range cruiser.
The car is comfortable, well-mannered, and not particularly tiring even
on a 900 mile day.

A long 75 mph cruise through moderate rain did not bring a single drop
of water through the convertible top. Wow!

The F55 suspension was supple and rode comfortably over a huge variety
of surfaces at all sorts of speeds, and particularly comes into its own
over about 75 mph. Only two stretches drove the C6 into uncomfortable
ride motions. One was a horribly broken and cambered stretch of blacktop
with thousands of small irregular sharp-edged patches that created sharp
suspension inputs in the range of 0.5" to 1.5" at 70-90 hertz. The C6
was harsh over this. So would have been most other cars. The other was a
stretch of short concrete slabs that seemed to be humpbacked. At 70 mph
these excited what seems to be about a 6-hertz natural frequency in the
front suspension and the car bobbed (pitched) steadily with about a 1/2"
to 3/4" amplitude. Adjusting the suspension back and forth between
'tour' and 'sport' did not affect this noticeably.

The C6 is not the quietest car I've ever driven but sound levels in the
ragtop are not a problem even on long hauls. On one stretch, 80 mph into
a gusting quartering 45-mph headwind, the noise level over the roof was
a bit intrusive after an hour or two. Aside from that unusual
circumstance, no issues with noise. Quieter tires would be nice,
especially over concrete.

Dad, you asked for fuel usage notes, especially as concerns different
brands of fuel. I measured fuel use per tank, per stretch of road or
driving conditions, and over a number of 5- to 10-mile stretches of
similar roads and fuel loads to measure effects of various factors of
fuel use. I varied fuel brands and usually burned the fuel down to about
1/4 tank before filling so results would mostly reflect the current
brand of fuel. All fuels used were premium 91 or 93 octane. Here goes.

The C6 averaged 28.0 mpg over 4200 miles of about 65% Interstate and the
remainder 2- and 4-lane US highways. Most driving was level to gently
rolling terrain with temperatures in the 50's through the 70's. I spent
several hundred miles plowing into a quartering 45-mph headwind at 75-80
mph and on that stretch fuel use increased sharply to about 22 mpg. If I
remove that stretch from the equation, the C6 averaged 28.9 mpg over the
remainder of the trip. Best leg was a 60 to 65 mph cruise over gently
rolling terrain on a two-lane blacktop at 33.4 mpg. Worst was pounding
80 mph into aforementioned headwind at about 22 mpg.

Five- to ten-mile spot checks for steady cruise on level smooth pavement
in calm air showed 33.4 mpg at 60 mph, 32.4 at 65, 30.4 at 70, 28.5 at
75 mph, and around 26 to 27 at 80.

Fuel burn varied both with brand of fuel and between loads of the same
fuel. Besides varying with speed (duh) it varied with traffic density,
terrain, and whether I used the cruise control. The challenge was
separating all the variables and figuring out how what really affected
fuel use. Here's what it comes down to.

Dad, there was no consistent, measurable variation between brands of
fuel, including 'top tier' fuels. Neither did I find any difference in
the car's driveabilty or performance. Variation between tanks of the
same brand was larger than variation between brands, one presumes due to
differing regional formulations and/or uncontrolled driving variables.

The C6's fuel burn consistently increased 6% to 8% when travelling in
the turbulent wake of heavy traffic at Interstate speeds.

Fuel use increased an average of 5% to 7% on cruise control over manual
throttle except on very hilly terrain, where the two were about even. On
dead level terrain manual throttle burned 3% to 5% less fuel than cruise
control. I can only surmise that the cruise control fiddles with the
throttle more than I do.

Rough pavement increased fuel burn 5% to 10%. Makes sense, it takes
energy to move the suspension around and that eventually is expressed as
drag.

-- Pete