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Determine Rear End RATIO on C5 Corvette's

Place your A4 (automatic)  into third gear (This also works with the six speed manual transmission using 4th gear) and get on the highway. At a constant 75 mph (making sure the TC has locked) read the rpm value; then divide it by 1000.
Examples:

2730 RPM........ you have a 2.73

3150 RPM........ you have a 3.15

3420 RPM........ you have a 3.42 (not a factory available ratio with the A4)(Standard available ratio on a 6 Speed)

3730 RPM........ you have a 3.73 (not a factory available ratio)

3900 RPM........ you have a 3.90 (not a factory available ratio)

4100 RPM........ you have a 4.10 (not a factory available ratio)

(Assuming rear tire height has not been altered from the factory height. Typical factory tire height on the C5 is 26.2 inches)