Kia make some pretty efficient cars now. They actually produce the most fuel efficient car on the market at the moment (excl dodgy stats from plug-in hybrids) - the Rio 1.1TDI. May not be available in the US, though.
I m glad your mother likes her car, but KIA Soul is not a bad small car at all. Happens to have a 5 star crash rating...which means a lot more to me than a few mpg since the mpg won't matter if I am dead. With a 3 star side crash rating on the mazda 3 I would have to already have my casket picked out.
I work for a Kia Dealership in the UK. Kia as with all new cars need upto 10K miles to loosen up and get more economical. But remember all manufacturers official MPG figures are done on a rolling road over 8 mins so they are just a guide.
All are subject to driving style, temperature outside and if its motorway or town driving.
I have had my 2014 Kia Soul Base Automatic for 8 months. I now have just over 11,000 miles on it. I'm working on entering all my fill ups on here. When I go on road trips and drive mostly highway, I get around 30-31 MPG, but in city driving, I get anywhere from 16-18 MPG. The city Rating for this car is supposed to be up to 24 MPG. To me a 33% reduction in city driving (Where I drive 80% of my miles) really sucks. I asked my dealer when I had my 7,500 oil change and they said I'm the only person they have heard of having such bad city mileage. Any suggestions? I drive very short distances.. From work to home is only about 5 miles.