All you have to do is document fuel purchases and mileage, you can then calculate mpg yourself. Everyone rides different, so their mpg will differ. There are apps for your phone to help (fuelly is one) if you have a smart phone.
That being said, your company is being way cheap. I get $0.51/mi no matter what vehicle I take.
1986 Kawasaki Vulcan 750
NGK Iridium Plugs #7803/DPR7EIX-9
Duralast Gold ETX15 AGM Battery
Coastered & Shaved
Metzeler ME880 [110/90-19, 170/80-15]
Balance Dampers Replaced
Rebuilt Forks w/ Progressive Springs
VN750.com Grill Cover
Emgo 23-92411 Handlebars
MOSFET FH012AA R/R
1990 Kawasaki Vulcan 750
1998 Honda VFR800 FI
2014 Honda VFR800F
1989 Pontiac Firebird Formula 350