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Friday, May 15, 2020

Gear Ratios : Speeds In Gears : When To Shift a Skyline GT-R





This post is all math. If you don't have some math, to fill into the blanks, you can't figure any of this out.  What you need to know:
  • Tire Height - can be calculated from
  • Transmission in the car
    • Is it stock?
    • Is it modified?
    • 5 speed
    • 6 speed
  • Overall Gear ratio
    • 4.111 is stock R32/R33 GT-R
    • 3.545 is stock R34 GT-R
    • It can be something else
  • Shift Point
    • If you shift with the stock tach you are shifting at 7600 rpm
    • If you shift at the limiter then that is most likely 8000 rpm
    • If you shift at a different point, the links are below to figure that out
 Gear 1 2 3 4 5 6
 R32/R33 3.214 1.925 1.302     1.000 0.752 
       
 R34 3.827 2.360 1.685 1.312 1.000 0.793

One example, my bluecar.  It has a 275/35/18 tire on it, which is 25.58 inches tall. It has a stock 5 speed transmission. Overall/Final gear ratios are 4.111. It is shifted at 8500 rpm. 


4.111, stock transmission, shifting at 8500 rpm.
4.111. stock transmission, shifting at 8000 rpm on the stock tach, aka 7600 rpm. 

So you can see that by shifting at a higher RPM, you fall into a bit better point on the dyno chart for this car, and this turbo combination.  https://www.gtrusablog.com/2019/10/finally-running-bluecar-on-dyno-results.html

So about if we swapped to a Getrag? Well actually the bluecar used to have a Getrag in it, but we sold it to a friend, that needed one.  So add a 6th gear, keep the 4.111 gear vs the 3.545 gear of the R34. Then we get:

5242, nearly that magic number. 

Now onto a sequential.  My Nismo has(had, till I stole it out for Big Bird 2) OS88 sequential transmission, and 4.375 gears.  255/50/16 tires are 26.04" tall. The shift points are higher at 9000 rpm, as this car has a lot of cam in it. The OS ratios are:

4.375 overall gears. 9000 rpm. 61 mph in first gear



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