How do you get from San Miguel de Allende to the Gulf side

We will be traveling about October and have never gone down the middle of Mexico. Have always gone directly down the Pacific side or the Gulf side. This year we would like to see some of the interior as far as San Miguel de Allende and then cross to the Gulf side and continue South. We have looked over our maps and just can't seem to find any good way to do that crossing. Does anyone have any experience or even if we have to cross to the Gulf sooner than San Miguel that would be okay. We just don't want to end up at Tampico (too many bad experiences there). Thank you for any ideas you may have.