Scott Dixon, who's been fastest on the track most of the weekend, managed to "win" the Firestone Fast Six edging out qualifying leader Marco Andretti...
On edit, ( bold type commentary is in retrospect).

***

Things are happening in the Firestone Fast Six too quickly comment on, but the top-of-the-chart driver has changed several times, I'm going to watch and enjoy the streaming video. 

(The "Firestone Fast Six" is a home run for the IndyCar Series, it's the best and most exciting method of determining the front of the grid in road and street racing I have ever seen. Scroll down to my qualifying worksheet and note the number of the fastest drivers that lost a lot of speed from their best efforts in practice, it's likely those were "strategic" losses. Last season this race was the 2nd road/street race IndyCar ever ran, and the 2nd using the "Fast Six" feature. Last year Dario Franchitti and Giorgio Pantano were at the fast end of the T&S charts all through practice, but Franchitti spun on his out-lap in qualifying and missed the "Fast Six" and "Pantano may have had the best run of the day but had a bad trip through the chicane and would start 13th (this quote from my P&Q coverage last year)." Both of those drivers tried varying pit strategies in the race trying to make up for their poor starting positions, but to no avail, Franchitti finished 8th, Pantano 13th. Dario was still talking about that on the radio feed after Fast Six qualifying yesterday, when all six drivers in the run with the exception of Marco Andretti lost significant speed while being careful but fast enough to make the Fast Six run.)

Once the Fast Six session ended it was Dixon on the pole, Marco Andretti came back to regain 2nd, and Castroneves gained a position over Kanaan late for 3rd.

Pos

Driver

 

Best Time

Best Speed

1

Scott Dixon (9)

 

1:17.0344

107.484

2

Marco Andretti (26)

 

1:17.0830

107.417

3

Helio Castroneves (3)

 

1:17.2127

107.236

4

Tony Kanaan (11)

 

1:17.2320

107.209

5

Dario Franchitti (27)

 

1:17.2893

107.130

6

Dan Wheldon (10)

 

1:17.5946

106.708

Here's the IndyCar qualifying result for those out of the Firestone Fast Six, Briscoe had a big gain in the result and just missed the fast six. (Once Briscoe got to run under the same conditions as the faster cars he showed how well he can road race with a not-too-stellar team, I look for him to contend in the race, Briscoe and the team improved with every session this weekend. 
Hornish is in deep trouble here, we saw last year with Franchitti and Pantano how hard it is to make up ground from a poor "Q" position, and they weren't under the pressure Sam will be under to loose the least amount of championship points and positions as possible today ).

Pos

Driver

 

Best Time

Best Speed

7

Ryan Briscoe (5)

 

1:18.2612

105.800

8

Vitor Meira (4)

 

1:18.3012

105.746

9

Bryan Herta (7)

 

1:18.3510

105.678

10

Sam Hornish Jr (6)

 

1:18.5940

105.352

11

Danica Patrick (16)

 

1:19.0287

104.772

12

Buddy Rice (15)

 

1:19.2128

104.529

13

Kosuke Matsuura (55)

 

1:19.2421

104.490

14

Jeff Simmons (17)

 

1:19.5633

104.068

15

Jeff Bucknum (14)

 

1:20.0960

103.376

16

Ed Carpenter (20)

 

1:20.3379

103.065

17

Scott Sharp (8)

 

1:21.6590

101.397

18

Tomas Scheckter (2)

 

-0.0001

0.000

This is what I expect the qualifying order to be. Road and street course qualifying the way the Indycar Series does it is like walking a tightrope, if you don't run hard enough and take a few risks, you'll find yourself out of the Firestone Fast Six, and the closest you'll start is 7th. On the other hand, if you press to hard and make mistakes like Franchitti and Pantano made last year, you're out of the Fast Six as well.

Indy Grand Prix of Sonoma Qualifying Worksheet...

Car

Driver                                                               

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********Firestone Fast Six*******
26 Marco Andretti (106.937) 106.831, this run will at least make the Firestone Fast Six.

***

********************************
3  Helio Castroneves (108.083) 106.597, another driver with a bobble off the carousel.

***

********************************
11 Tony Kanaan (107.285) 106.586, watching it on live video stream it looked faster than it turned out.

***

********************************
27  Dario Franchitti (106.942) 106.519, one bobble into the rumble strips, but in the Fast Six.

***

********************************
9 Scott Dixon (107.818) 106.507, Dixon too with a big drop in speed, probably just being careful knowing the speed he needs to beat

***

********************************
5 Dan Wheldon (107.253) 106.203, another lost mph.

***

********Firestone Fast Six*******
5 Ryan Briscoe (105.189) 105.800, a very good gain over his best previous run.

***

********************************
4 Vitor Meira (106.752) 105.746, lost a full mph.

***

********************************
7 Bryan Herta (106.951) 105.678, another of the faster drivers losing a lot speed.

***

********************************
6 Sam Hornish Jr. (106.524) 105.352, Sam had a couple of bobbles on the course, and IIRC he will mist his first Firestone Fast Six

***

********************************
16 Danica Patrick (104.196) 104.772, her best lap of the weekend... 

***

********************************
15 Buddy Rice (105.466) 104.529, almost a full mph slower than his best effort.

***

********************************
55 Kosuke Matsuura (104.196) 104.490 with the exception of Sharp, qualifying in reverse speed order has produced a new best speed.

***

********************************
17 Jeff Simmons (104.334) 104.068, no surprises here.

***

********************************
14  Jeff Bucknum (103.664) 103.376, about his practice best, he'll at least start no further back than 15th.

***

********************************
20 Ed Carpenter (102.702) 103.065 mph, a good gain over his fastest lap of the weekend.

***

********************************
8 Scott Sharp (103.756) 101.397, Sharp is a good road racer, but he has been off pace all weekend, but this poor result was due to a problem on the course during the run.

***

********************************
2 Tomas Scheckter (104.310) Engine troubles, Tomas may have been called in by Honda based on telemetry. Because of that it appears that Scheckter will get another lap.
(An Update) Scheckter wasn't allowed a run, just made a lap to confirm the problem was fixed

***

********************************
The morning practice session has ended with Helio Castroneves posting the first 108-mph lap of the weekend. Scott Dixon has the TCGR Panoz in 2nd, with teammate Dan Wheldon 4th, bracketed by all four AGR drivers. Sam Hornish has fallen to 9th quickest despite being only slightly slower than his best speed of Friday. Buddy Rice has finally cracked the top ten on this ICS T&S chart. 

Here's the schedule for the remainder of the day:
 Qualifying Sat, 26 Aug @ 3:05 p.m., Don't forget to listen to Mike King on the IMS Radio Network feed beginning at 2:45 EDT.

Final Practice Sat, 26 Aug @ 9:20 p.m.

Pos

Driver

 

Best Time

Best Speed

1

Helio Castroneves (3)

 

1:16.6075

108.083

2

Scott Dixon (9)

 

1:16.7958

107.818

3

Tony Kanaan (11)

 

1:17.1773

107.285

4

Dan Wheldon (10)

 

1:17.2008

107.253

5

Bryan Herta (7)

 

1:17.4183

106.951

6

Dario Franchitti (27)

 

1:17.4253

106.942

7

Marco Andretti (26)

 

1:17.4290

106.937

8

Vitor Meira (4)

 

1:17.5629

106.752

9

Sam Hornish Jr (6)

 

1:17.8841

106.312

10

Buddy Rice (15)

 

1:18.5084

105.466

11

Jeff Simmons (17)

 

1:19.3607

104.334

12

Tomas Scheckter (2)

 

1:19.4627

104.200

13

Ryan Briscoe (5)

 

1:19.6739

103.924

14

Jeff Bucknum (14)

 

1:19.8943

103.637

15

Danica Patrick (16)

 

1:20.0510

103.434

16

Kosuke Matsuura (55)

 

1:20.1785

103.270

17

Scott Sharp (8)

 

1:20.8707

102.386

18

Ed Carpenter (20)