In Some Tasker Profiles I detail how to set up a profile to read out a SMS while driving. As I need the same contexts I will simply add a task to the profile.
A quick reminder of original profile
Event: Received Text
State: Power [ Source:Any ]
Application: CoPilot Live EU Major
Wait [ Seconds:4 ]
Say [ Text:%SMSRN sent the following SMS: %SMSRB ]
If I'm driving for a long time, I'd always have the phone plugged into a charger and on copilot. The reasoning behind the order of the contexts is predominantly for the sake of battery life (minimising the context checks). The delay in saying the SMS is to allow the notification tone to finish.
The extra task you need for auto response
Send SMS [ Number:%SMSRF Message:[Auto Response] I am currently driving and will get back to you soon. James Store In Messaging App:On]
I have formatted the above to make it more clear. Whether or not you 'store in messaging app' makes no difference to sending the message, I simply wanted a record of what has been sent back and forth.
You can download the full profile Read SMS While Navigating 2, don't forget to change the name if you're not called James!
On Android Market: