Why don't girls feel comfortable when they get a ride in my car ?

i'm somewhat confused by this behaviour , i've had 2 very similar encounters with girls after leaving a bar/party . i don't drink much like 1-2 beer but still go out to bar so often drive my car as i live outside of town and no other way to get in .
anyways been 2 times i have given girls a ride in car and each time things just got weird and they start making comments worrying that something bad is going to happen , what if he does this to us etc and they seemed really out of line and if they trusted me that little why did they get in car to begin with .
now the first encounter involved 2 girls from back home and i knew one them fairly well , gave them a 10 minute ride to a gf's house after leaving the bar one night and they got annoyed when i took a shortcut down a sideroad instead of the main highway .
the second encounter happened last night as i was leaving a bar in a small city , 2 girls who were also at same bar but that i didn't really know asked me for a ride as they couldn't find a taxi , so i gave them a ride also maybe 10 minutes or less . and when they first got in all kinds of weird comments and such , if you try anything etc . but they felt better as ride continued and seemed to like me once i dropped them off .

i'm confused by this as car is a like new 2014 chevy cruze , windows are not tinted , no loud scary music , car is clean , i'm not a big guy , don't smoke in car and no alcohol bottles , and both times it was a pair of girls so they weren't even alone in car . it to me seems like a really safe looking car and atmosphere to be in .
Why don't girls feel comfortable when they get a ride in my car ?
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