4200 miles in six days in a C6 - fuel use notes for Dad

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
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Was this all done with the new 6 speed automatic? If so what did you think of the new transmission and the "paddle shift" on the steering wheel?
Glad to see you made a safe trip and thanks for such a concise report.
--
Dad
05 C6 Silver/Red 6spd Z51
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Dad wrote:

Oops. The car's a six-speed stick.
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What is the stated shutdown procedure for the '06, C6, 6 speed?
Kinda hoping that leaving the gear shift in any gear except neutral will shut down the power. Next is that it is a software change and the '05 can be updated.
Thanks
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I'll bet you aren't the only one hoping for that!!
Dad wrote:

--
Ric Seyler
Online Racing: RicSeyler
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Well the people that make PreparationH would have a sharp drop in stock prices. It has been ok but still a pain in the ass.

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I'm so anal it would bother me just to have my car sitting with the shifter in reverse. It's gotta be in first, just for the aesthetics man!! :-)
Dad wrote:

--
Ric Seyler
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thats where the surgery would benefit. then ya have a "perfect ass: :-)
just kiddin
--
"Key"

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...... then he wouldn't be able to keep his hands off of himself even more than usual, one hand would be busy in the front and now the other hand would be busy in the back.... and now how in the hell would he be able to drink a beer!! LOLOL
<ahhh a beer helmet> DANG answered my own query.... :-)
'Key wrote:

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Ric Seyler
Online Racing: RicSeyler
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Pete,
Very nice posting. I enjoyed your comments
Thanks
aRKay 01 BK/bk/6-speed Coupe
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