I agree with threejs.
A mother-daughter relationship can be soo special. If it's her gas money and not yours, dont worry about it. She obiously loves her mom very much you shouldnt get in between that it will only mean trouble.
Though talk to your girlfriend and tell her how this is affecting you. Maybe you two can come to some kind of comprimise. However, if it were me, if my fiance of 5 years came to me and said stop talking to my mother as much, who is my best friend in the world, I would kick him to the curb. He is very understanding and actually loves the relationship I have with my mom because he thinks that makes me even a better person and eventually may make me a great mom as well. http://www.healthboards.com/ubb/love1.gif
Nothing comes before Mom. Period.
I know its frusterating but it's something of her you will have to either tolerate or not. Hopefully you come to a comprimise. Maybe explain to her how you think that she needs to concentrate more on the future life with you,etc.
All this is just my opinion.
Take care I hope you two work things out
------------------ I wish to be the kind of person my dog thinks I am.