I have a feeling you are asking so that you can beat the high stakes challenge for each tier's crew boss huh?
For those that don't want to know what will beat them crew bosses' high stakes challenge stop reading.
This is for version 1.0.7:715 I just beat tier 4 and got the boss car for all the previous tiers. I made sure my cars were fully upgraded and used the mechanic tune up for a few gold, but not the that extra "boost" for 2.99.
My cars (fully upgraded): T1: Nissan Altima Coupe (Rated 352) T2: Chevrolet Camaro SS (449) T3: Chevrolet Corvette C6 (535) T4: Audi R8 V10 FSI (628)
I got each of the boss cars and will post their car ratings for when you race against them in the high stakes challenge. This rating will also be the car you get if you win that high stakes challenge.
T1: 348 T2: 443 T3 547 T4: 624
END OF SPOILERS.
I really wished this info was posted so i knew if my fully upgraded car was fast enough to beat the high stakes challenge... --- D2x w/sc/l and nl *zombiejehuty *awong2000 *alberta-ss