Pretty sure it should be "grandma-ia".
Remembering whether the gender of the verb depends on the person or the object they're acting on is entirely beyond me.
Although, the modern mini was designed by Americans working for BMW, and is also manufactured in other countries.
I also recall that on one of their models, they consistently stuck the back door on the right side, so if you were in the were in the back of one in Britain you had to get out into...