It would make it quite hard to drive a 1-person vehicle. Now that single person would have to drive, gun, and shift. It's all ready pretty hard just driving well and not running into trees as your firing at infantry.
The real advantage of a manual car is you can usually rev it higher, giving you better acceleration if you know what your doing and also, they can reach higher speeds, but only by 5 or so MPH.
It's not really needed in PS.
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