In most cases, Dash should automatically take care of choosing your car. However, there are some cars which make this harder for us by not reporting their Vehicle Identification Number (aka VIN.) Most of these are from before 2000, when the OBD2 standard was still young.
Vehicle Profile Page
If you have two cars, and only one of them reports its VIN via the OBD, change your default vehicle to the one that doesn't report its VIN. You can tell when this is the case because we'll pull in the VIN to your cars' vehicle profile page in My Garage.
One last note: deleting a car will also delete all of the associated trips. We recommend against this in most cases.