If you have access to a slide-rule, you can use the C1 and D scale to calculate maximum speeds for different gear ratios.

Simply line up the maximum speed on the D scale with the gear ratio on the C1 (inverse scale--this requires using the cursor). The ratio used can be any of final drive, gear ratio, or the "effective ratio" product, so long as it's the inverse or denominator value such as used in the [size=+1]GT[/size] series. Your slide rule is now a chart which (as long as you scale correctly) reads on the D scale the maximum speed achievable (assuming sufficient power) if the chosen ratio is changed to the value chosen on the C1 scale.

When reacquainting myself with this, I found it convenient to use my circular slide rule since the ratios often do move their decimal point.

For instance, line up 95 on D with 1.64 (the value of the previous gear in the line). The D@C1 scale then reads 49@3.18, 72@2.18, 126@1.24, 155@1.01, (confirming the other figures in the chart) and... 168@0.929.

So at 8200rpm, we should be going 168mph in 6th gear.

So let's line up 168 on D with 4.29 on C1 (our final drive). 2.700 actually lines up with 266mph, suggesting other things are slowing the car down at that point. That calculation is actually confirmed by the in-game chart which shows a speed of 247 for getting into 6th (maxing 5th), and also observations of engine speed in 6th gear. (Though, in fact, when observing the tach, I have difficulty believing the speed at 8200rpm in 6th isn't a little over 168).

240 on D actually lines up with about 2.98 on C1, so let's try a final drive of 2.98, and see if it makes things any better or worse.

It seems to make things slightly worse. Now why would that be? Well, we need to look at the power curve (Change Parts: NA Tuning) with stated figures of 439.7ft-lb@6000rpm Max Torque, and 568hp@7000rpm Max Power. We really should not be trying to drive the engine up to 8200rpm at high speed. In addition, and related, it leaves us with only two gears, instead of three, to use as speed drops around the banking.

Roughly speaking, when you change up gears, you want to end up at the maximum point on the torque curve, but, in fact the lower gear will have more effective torque for given torque, and so you'd compare the torque in the gear you are in to T2*G2/G1 at rpm you end up, and shift when that latter figure is higher (i.e. when you've gone so far past the point where the torque curve drops off that the real torque loss caused by the higher ratio is offset by the torque gained by moving to the new point on the curve).

T2 = torque figure in new gear (at new rpm)

G2 = gear ratio of new gear (smaller denominator value)

G1 = gear ratio of current gear (higher denominator value)

Note that the torques in question will be at different points on the curve (corresponding again, in fact, to the ratio of the gear ratios; also there will be rev loss caused by shifting).

Last edited: Mar 28, 2009