sudden 5 mpg drop on computer dash readout

can anyone think of reasons for a sudden 5mpg drop in computer readout (immediately following 40K service check)...for 2000 passat AWD wagon.....
used to start out hway at around 15... and cruise around 25-7mpg, now 19, and 20-22! thanks... J
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
from what i've noticed, when time comes close to an oil change, the gti gets better slightly gas mileage (from 300 miles on 13 galons to 320 on the same amount). maybe something to do with less oil to slush around. right after the oil change, it goes back to normal. if you're really worried, go get your codes scanned at the dealer. the ecu will say if there's something wrong...
did the dealer fill your gas tank? maybe they put in 91 instead of 93 or something stupid like that?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
thanks... J

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.