I have my sump in the garage and my wife parks on the other side but it never smells like exhaust in the garage. Wife knows she has to get situated in the car and open the door before starting the car and pulling right out. When she comes back she will leave the door open for a few minutes to air out as long as the weather isn't bad. I also have a ventilation fan pumping house air into the garage, that's more for temperature control but helps with keeping car fumes vented as well.
If you are doing a frag tank and can't get rid of the smell in the garage I would cover the tank. I would either not run a skimmer, run carbon on the air intake, or run the air intake to somewhere with cleaner air.