Wow I feel like DP on this subject something I know a little about)
I have had 3 TDI's in the past 9 years (Diesel VWs -
WWW.tdiclub.com for those who thinks spark plugs are a thing of the past
)
Anyways, I have been tracking my fuel purchases and mileage for about 6 years now for the VWs (spreadsheet based - weekly fills all tracked (Heck I have even done a ventectomy on my tank neck in order to be able to fill my tank to the very brim in order to achieve 700 MPG (I AM Anal about it).
So here is the deal. With my '05 Pump Dusse Diesel Golf I get the following mileage driving on my daily 90 mile R/T Commute mixed route 80% HWY and 20% Rural back roads average about 60MPH overall which is 45 MPH on the back roads and 80-85 on the highway (until I hit the backups then I go 2MPH)
Summer Mileage:
No Roof Rack - 44.2 MPG measured weekly between 2005 and 2007
Yak Rack with 2 Steelheads & Kayak Saddles (turned down) - 39.3 MPG
Above Rack with 44" Fairing - 36.7 MPG Measured on and off over several months in 2006 and 2007.
So a roof rack drops me by 11%, A Fairing by an additional 4% and having 2 bikes on is worth about a 20% drop in economy for a diesel.
I differentiate between summer and winter mileage due to the lower Cetane Rating on Diesel in colder months.
I don't really like the fairing or the roof rack because of the noise level(though the fairing cuts down on the whistling and loud "rushing" sounds), but I like to bring my bike into work on nice mornings and ride for an hour.
Oh Yeah economy with 2 bikes on (Measured back and forth between Lehigh Valley and Buffalo or Hammondsport Area) is approx 35MPG.
PS Halia, I used to get approx 46MPG with my old A4 Golf I wish I never got rid of it.