Alabama is one of those odd states that has numerous types of transactions taxed at rates lower than their general sales tax rate. The regular sales tax rate on most items is among the highest in the U.S.- parts of Birmingham located in Jefferson County have a 10.0% sales tax rate. The total sales tax consists of three parts- state, county and city tax rate. Autos are taxed at a much lower rate than most other transactions, with a state rate of only 2%. If you actually live in Mobile, there's a 1% Mobile County rate and 2.25% more for the city of Mobile. That adds up to a total fo 5.25% if you register the car in Mobile. If you live in an outlying county near Mobile, the rate will probably be a little different.
I live in Georgia but I'm actually in Alabama this weekend visiting my best friend. I took a look at the rate he paid on the Lexus IS250 he purchased last year and it was only 3.375%. The breakdown for that was 2% state, 0.375% for Shelby County and 1% for the city of Calera.
Florida, like Georgia, has a flat sales tax rate for all merchandise- a new car is taxed at the same rate as a replacement wiper blade or almost anything else. Everything either gets taxed at the same rate or is tax exempt, nothing in between. And the only exempt items are usually groceries and items for resale.
Florida has a state tax of 6% and most counties have a surtax in addition to the state rate. For most counties, it appears to be 1% (Duval, Miami-Dade are both 1%, for example) and some are only 0.5%. A few such as Brevard and Broward counties have no surtax. Here's a link to those county rates in case you might need it-
And here's the document which explains to Florida car dealers how and what they're supposed to collect and report- http://dor.myflorida.com/dor/forms/2009
So you'll most likely pay 7% in Florida and Mobile woud be 5.25%. BUT the address where the car will be legally registered determines the sales tax rate you'll pay. If you plan to register the car at the address of your parents or other family in Florida, you'll pay 6% plus the county surtax if there is one. If you plan to register the car in Alabama, which requires an Alabama driver's license and proof of residence (usually a lease or copy of utility bill in your name), it will vary based on the city and county.
One remaining question is WHO collects the sales tax in each state??? In Florida, they require you to sign an affidvait and have it notarized stating that you are a resident of Alabama and plan to register it in Alabama within X number of days. They will then collect the 2% Alabama STATE portion of the sales tax. You would still be required to pay the county and city sales tax at the time of registration or before. You may have to pay the county their part of the tax then visit city hall to pay the city their tax. if you buy in Alabama and register it in Florida, I don't know if the Alabama dealer will collect the Florida tax or if you would pay it when you register it in FL....
A few years back, I was shopping for a new Mazda3 and the color and options I wanted was nowhere to be found in the state of Georgia. I ended up finding it in Knoxville, TN and drove four hours each way to pick it up. Since I was a Georgia resident and the car would be registered in Georgia, they didn't charge me either state's sales tax. It was my responsiblity to schedule an appt. with the Georgia DOR to pay the sales tax prior to registering the car in my county. It was a hassle for me, but it seems far less complcated than what I've described above....
I hope this helps.....I wrote it all, but I'm not sure that I understand it completely! =)