got curious. your inside mirror is sufficient if it meets criteria.
you can find the rules here:
http://www.txdps.state.tx.us/vi/insp.../item_insp.asp
some tasty excerpts:
20.19 Mirror. Every motor vehicle shall be equipped with a mirror so located as to reflect to the driver a view of the highway for a distance of at least 200 feet to the rear of such motor vehicle.
2. Inspect for and reject if:
b. Vehicle is not equipped with at least one mirror.
NOTE: An inside mirror would meet all the above requirements. If the vehicle is equipped with more than one mirror, only one, either inside or outside, needs to meet all requirements.