Is percentage ambiguous?

I hear statistics such as “this provides a 200 percent increase over the current gas mileage.” If your gas mileage was 10mpg before the increase, is the improved mileage 20mpg? or 30mpg? 20mpg is 200% of the original mileage, but 30mpg is a 200 percent INCREASE (20mpg) over the original mileage (10mpg) for a total of 30mpg. I’m good with math but I have always found this type of statement confusing. Is it just me? If your heart beats 100 beats per minute and changes to 110, would you call that a 10% increase? a 110% increase? An increase to 110%? Any and all of these?