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* 2004/2005 GTO's top speed is electronically limited to 155 mph
Engine RPM: To calculate top speed, you must type in the maximum RPM value that your engine will turn. The RPMs available in the factory drivetrain information help screens (except for the 2004/2005/2006 GTO) are based on the RPM value at the rated (net or gross) horsepower of the engine as specified by Pontiac. This may not be the maximum RPM value that your engine will turn. To check the calculation against your car, try a lower value like 3000 RPM and go out and really try it.
Trans Gear Ratios: For a top speed calculation on older GTOs, the gear ratio is really 1:1 for the highest gear. Unless someone stuck in an automatic with overdrive. This was mainly placed in this sheet to tell you what speeds you could obtain in different gears. The torque converter is assumed to be 1:1 for automatic transmissions.
Rear Axle Ratio: This one might be difficult to obtain. Differentials are labeled and/or tagged with a code. You can look this code up to get the ratio. The other method requires actually spinning the wheels and counting revolutions. Block the front tires of your GTO. Jack up the whole rear of the car. While turning the tire ONE full rotation, count the number of times the drive shaft turns. A chalk mark can be used as a reference point. If you have a posi (Safe-T-Track) rear, then the number of turns of the driveshaft (per single wheel rotation) is your gearing (i.e. 3.50 turns would be a 3.50 rear, or a 3.55 more likely).
If you have an open (non-posi) rear, then you must multiply the number of driveshaft turns by 2 to get the gearing (i.e. 1.5 turns would be a 3.0 rear [1.5 * 2 = 3.0], or a 3.08 more likely). The reason for multiplying by 2 is that in an open differential, the planetary gears cause the driveshaft to rotate half as fast as in a posi if only one tire is being rotated. Therefore, you must multiply the drive-shaft turns by 2 to get an accurate ratio, otherwise you must rotate one tire then the other (and not multiply). When one tire is not moving, the planetary gears are "walking" around the differential housing in the opposite direction causing the drive-shaft to rotate slower than if the diff were "locked".
Rear Tire Size: The rear tire diameter is calculated from it's size code. My GTO has 215/70R14 tires. Tire side-wall height (distance from ground to rim) is calculated by converting the tread size from 215 mm to inches by dividing by 25.4. Then multiply by its aspect ratio 0.70. The radius of the tire is found by adding half the rim size, 7 inches, to the side-wall height, 5.9 inches. The circumference is 2*PI*radius.
Old GTOs came with tires with different codes. I converted these codes to the current code types. Although some of these codes are not available as I converted them, it's important that the circumference is calculated correctly. This is true for both calculating Top Speed back when the car was sold as well as keeping an accurate speedometer with today's tires. The reference I used for tire size conversions can be found at tirerack.com.
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