The drag mode differs from that in NFS: Most Wanted. In Pro Street, the player has to burn the tires so that during the drag race, the tires offer maximum grip to prevent any wheel spin.
The drag length is 1/4 mile and 1/2 mile. The best car for this mode is a car having the best acceleration. That is, choose a car having the minimum time achieving the 0-100kmph speed range. Tune the car so that it has a better aerodynamics. Rear wheel drive cars are known to have better grips as during the acceleration the weight of the car is shifted to rear end and the wheels at the end do not lift off the ground preventing wheel spin.
The fastest I have been able to go is a 6.94 in a Mustang GT500 (the new one) Tuned as follows
All upgrades at stage 3 Widebody kit and the drag spoiler (it's the last choice in the list)
Tune your suspension as follows softest spring settings for the rear springs and softest damper level in the front This allows it to hookup better
Make your gearing set to the tallest possible settings for all gears and the final drive
Advance your cam timing 3 ticks off the max setting Max boost settings Make the flow the lowest and max the pressure
Have fun running into the 19's