I don't have an exact speed because this included all city and highway driving.  I followed the basic rules, don't exceed the speed limit but I did on occasion exceed 55 if I determined that safety may be an issue.
I made a tracking form in Excel, but since I couldn't attach it, here is the data in text form.

       5/9/2009        5/16/2009       5/23/2009       5/28/2009       6/4/2009

Miles   397     412.6   408.7   405.5   440.1   2063.9
Fuel/Gal.       10.09   10.11   9.844   9.893   10.413  50.35
MPG     39.3    40.8    41.5    41.0    42.3    41.0
Cost    $23.61  $24.66  $25.58  $26.11  $28.10  $128.06
Price/Gal.      $2.34   $2.44   $2.60   $2.64   $2.70   $2.54

Base MPG        33      33      33      33      33      33
Base Fuel       12.03   12.50   12.38   12.29   13.34   62.54
Base Cost       $28.15  $30.50  $32.18  $32.43  $35.99  $159.07
Savings         $4.54   $5.84   $6.60   $6.32   $7.89   $31.01
% Cost Reduction        16%     19%     21%     19%     22%     19%

Tin Cheung, TX

• Greater fuel efficiency
• Reduced emissions
• Improved highway safety.

The trip to Honesdale from Ossining is estimated by Google maps to be 108 miles and about 2½ hours in duration. I took this trip with my wife this summer to attend a wedding, and it took us about 30-40 minutes longer than the google estimate. The reason being that I tried to travel at least five miles below the speed limit on every road and travel at 45 mph on I-287 and I-84. Let me assure you from the start, the idea of traveling at 45 mph on the interstate did not work. Even at 55 (the new upper limit I set on the trip) it felt scary when semis, at their 75-80 mph speed and tens of thousands of pounds of mass, bore down on our little Toyota Corolla.

I am a 27 year old male born and raised right here in Sacramento. The overwhelming social influence I was subjected to during my youth was that environmentalists were invariably card carrying socialists and anything that wasn't a V8 with a manual transmission was not only un-American but was solid grounds for questioning someone’s manhood. My father saved me from all of that by imbuing me with the foundations I needed to question the norm and develop my own understandings of the world. But it still took me a while to shed the socially imposed shackles and raise my head above the fog.

My first car was a 1984 Corvette that I had for only two months, the second was a 1989 Camaro RS. I had an irrational need for speed and was only under 55 when at the pump or in the driveway. I joined the military at 19 and soon after that I purchased my third car, a 1994 Corvette ZR-1, this didn’t help my speed problem.

I've been doing the 55 mph routine long before I found this site.  I've owned hybrid cars with the built in mileage computers, and had made it a game to push the graphics ever higher, and log ever higher figures into my sheets. I no longer have the hybrid cars, but still keep to the 55 mph rule.  Case in point, my current ride, which is a 2008 Chevy Impala 2LT with the 3.9L V6 that cuts down to three cyilnders under low load conditions.  Tonight on the way home, I've posted a new high number - 33.8 mpg by the car's computer.  This isn't via a solid nonstop highway drive either. I have about 15 traffic lights on my 50 mile commute to get through, and three of them still managed to get me to come to a full stop, despite the late night hour that is my drive time home.  I have a photo of the car's mileage computer as proof: 

Bryan Foust
Paradise, TX

EPA MPG rating on this car is 18 mpg city, 29 mpg highway, 22 mpg combined
http://www.fueleconomy.gov/feg/noframes/24696.shtml

Update, 10/14/2008

I am writing tonight to give an update on my efforts to push my fuel economy to new heights on my 2008 Impala.  Tonight, not only did I beat my old record of 33.8, I practically shattered it.  I pulled into my driveway after my drive home from work, to find the mileage readout posting a new high of 34.9 mpg.  That is not all, for a while, it was showing as high as 35.3 mpg, before the hills in my immediate area took their toll on the numbers.  As with my first email, I have a photo of the screen: 

I don't care that the price of gas has been coming down as of late. Maintaining my rule of driving no faster than 55 mph, combined with the recent downward trend in prices at the pump (I will not call it "cheap" untill we see pre-Katrina prices, and I'm not holding my breath for that) both combine to further help give my fuel budget a bit of a break.
Bryan 

Update January 4, 2009

Once again, I write back to share another experience of keepting to a maximum speed of 55 mph.  I recently just returned from a holiday visit to family over the Christmas and New Year's holiday.  The drive up to Colorado included a battle with headwinds, and resulted in a none too remarkable 32.3 and 31.7 mpg respectively out of the two tanks of gas purchased for the approximately 750 mile drive.

My drive home however, would prove to be much more fruitful.  The temps were warm (almost too warm) and winds more favorable for the return trip. I topped off the tank before heading down the highway, and set the cruise to the requisite 55 mph speed.  As the miles ticked by, the computer kept gradually climbing higher, as the "AFM" system in my Impala took every possible opportunity to turn the V6 engine into a V3.  By the time I entered into the Texas panhandle area out of New Mexico, the mileage computer was solidly locked into the 36 mpg figure.  Still the computer would notch upwards a couple of more ticks by the time I began to see Amarillo on the horizon.  Needless to say, I was thrilled to see my full size car enter into fuel economy territory occupied by the Chevy Aveo.  As Amarillo was my planned city to stop for gas, I pulled into the Pilot Truckstop located at I-40 and Loop 335.  When I shut off the ignition, the computer was showing 36.5 mpg.  As with all of my record mileage claims, I've included a picture of the computer:
 http://farm2.static.flickr.com/1247/3167759237_0ea17d54c4.jpg

This number was obtained after a 437 mile drive.

The rest of my drive home would still prove the merits of keeping my speed to 55, and letting the car's cylinder deactivation make the most of itself.  The continuing drive down US 287 to the DFW area includes lots of little towns to slow down through.  The mileage computer still posted a very respectable 35 mpg, despite having traffic lights to contend with in the small towns.

I am a retired police officer and was working the east side of Tampa during the first mandatory 55 mph limit.  Prior to this, we experienced about 2 fatalities per week on I-275 and I-4, a lot of injuries and extensive vehicle damage. After the reduced speed (as they said, strictly enforced), we dropped to about 2 fatalities per month, a reduction in injuries and property damage.  I know that people are bored while driving at 55 but consideration for the safety of others must come before being entertained while driving. Speed does kill and too often.

Al Ratliff Jr.
Retired TPD Corporal

Thank you Officer Ratliff! More about speeding and safety is published in the Safety section of this website.

All you need to know.
W = F X D
Work (energy consumed) = Force (aerodynamic parasite drag) X Distance traveled.
Aerodynamic parasite drag force increases as the Square of the increase in speed.

From 55 mph to 80 mph the speed increase is 1.45. 1.45 squared is 2.11
Over DOUBLE THE ENERGY USED!

Conversely slowing from 80 to 55 (55/80 = .6875   Square this = .473 used)
cuts the energy, fuel burned to less than 1/2.

Jim Messina
Allentown, PA