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