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It is not miles per gallon but total gallons used.
At the end of the year, total your miles driven, including your bike or other vehicle and then total the gasoline used. Calculate the true miles per gallons.
I am reluctantly sacrificing my Chrysler Minivan (22 to 26 mpg after 200k miles) for a bike, a Suburban and a '88 Camry. Total cost $2600. I also bought a 12 foot enclosed trailer for serious hauling. This combo is pretty flexible. The Suburban, which looks brand new, will see less than 5000 miles a year, has less than 100k on it when purchased and cost $2500 during peak oil. The Camry has less than 70K and was free.
My buddy replaced his Volvo with a Prius and loves it. His wife wants to drive the Honda minivan into the ground. Different strokes.
The goal is to use less gasoline while meeting your needs. I always liked recycling older cars, keeping the total cost of transportation low.