Well the T50's 4th gear ratio is 1.00 and the Celica Supra's final drive is 4.10 so not calculating in wind speed and aerodynamic losses 5000 RPM is about 82mph with 185/60/14 tires .
8000 RPM in 4th without windspeed etc. is about 132 MPH
Assuming the same in 5th with the T50's .86 5th gear ratio 8000 RPM is 153MPH ...
Hope that helps
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