You are mistaken. The speed is unchanged by the gear ratio.
In your calculation you do not have tire diamter which does affect the gear ratio.
Let's just say though that in your example car it has the gearing to reach 115mph in 4th gear at 6500 rpm, but not the power to reach that 115mph. The car will NEVER reach 6500 rpm in 4th.
The gear ratio to speed is a fixed number.
In my example about the VR4 it has the gearing to hit 208 mph at 7300 rpm in 5th, but not the power. That means it will not hit 7300 rpm. Most guys have achieved a top speed of ~170 mph. That means they only hit approximately 6000 rpm.