Mike Mercury

our 2002/carcover/ramps/back to main/sound reduction/ham radio/3.42 upgrade


Determining your C5 A4 rear axle ratio 


All factory C5's with the manual transmission came with the 3.42 axle ratio, there was no other option available. But the A4 equipped models had two choices of axle ratios. The standard equipment was the 2.73. An optional "performance ratio" G92 was available as an option; this being a 3.15 ratio.


For those with A4's not knowing which ratio is installed in their C5 Corvettes, you can:

A. Either crawl under your C5 and read the printed ratio on a metal tag located just above the fluid fill location (not the bottom drain plug) of the rear carrier.

B. Or, look at your option list. If you look in your glove box there is a sticker with all codes on the inside of the glove box door. The 3.15 performance axle ratio option is code G92. For '99 and earlier it should be listed on the underside of the left-hand storage well cover:

GU2 2.73 ratio

G92 - 3.15 ratio

GU6 - 3.42 ratio (maunal tranny only)

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



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)

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 heigth on the C5 is 26.2 inches)


The key is third gear, driving at a constant 75mph. AND, you must be using the same rear tire height as the factory installed tires. It may be up to 100rpm off depending on the amount of slip your TC has when locked.

With this method, you don't need a chart to compare your rpm/mph to. Just divide the RPM by 1000. This also works with the six speed manual transmission using 4th gear (again with identical tire height as from the factory).


FYI: final overall ratio of an automatic A4 C5 in each gear:

2.73 - 3.15 - 3.42 - 3.73

8.35 - 9.63 -10.4 -11.4 (1st gear A4)

4.42 - 5.10 - 5.54 - 6.04 (2nd gear A4)

2.73 - 3.15 - 3.42 - 3.73 (3rd gear A4)

1.91 - 2.20 - 2.39 - 2.61 (4th gear A4)


 individual transmission ratios (by gear):

1st 2.66 
2nd 1.78 
3rd 1.30 
4th 1.0 
5th .74 
6th .50 
Rev. 2.9 

1st 2.97 
2nd 2.07 
3rd 1.43 
4th 1.0 
5th .84 
6th .56 
Rev. 3.28 

1st 3.06
2nd 1.63
3rd 1.00
4th 0.70