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I have long believed due to some forum articles, that driving 70 Mph in 4th gear was the way to determine rear gear ratio. I just checked my 2002 C5 again and it is nailing 3100 RPMs.
Now, everywhere else, I am seeing people say the 6 speed was always shipped with a 3.42 rear. Which is which?
If I have a 3.10, would an upgrade to 3.42 be a good idea?

Thanks
 

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The automatic trannys in the C5's came with a 2.73 rear. You could option it with a 3.10 rear. On manual trannys, they came with a 3.42 rear.

If your car came with a 3.10, upgrading to a 3.42 would improve your acceleration but the engine would rev at a slightly higher rpm.
 

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Welcome aboard the Good Ship Z06 :z::z:

3100 RPM in 4th gear (1.00) with either MN12 or MN6 Transmission with stock tires equates to a 3.42 stock Axel
 

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Welcome to :z:
Above comments :thumb:
Do you have a C5 LS1(MM6) or C5Z LS6(M12)?
You didn't ask about other gears, but transmission ratios are different:
Compliments of a FRC Tom post on an older thread:
Gear Ratios C5
LS1/MM6 vs. LS6/M12
1st Gear 2.66:1 2.97:1
2nd Gear 1.78:1 2.07:1
3rd Gear 1.30:1 1.43:1
4th Gear 1.00:1 1.00:1
5th Gear 0.74:1 0.84:1
6th Gear 0.50:1 0.56:1
Reverse 2.90:1 3.28:1
 

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The automatic trannys in the C5's came with a 2.73 rear. You could option it with a 3.10 rear. On manual trannys, they came with a 3.42 rear.

If your car came with a 3.10, upgrading to a 3.42 would improve your acceleration but the engine would rev at a slightly higher rpm.
There is no 3.10 gear for a C5. The C5 automatics shipped with a 2.73 OR the "performance" differential option which was a 3.15. All C5 M6s shipped with the 3.42. Off topic the C5 Zs shipped with a slightly upgraded (shot peened) 3.42 which has a different part #.

Oh, and the trick for finding out the gear ratio with an automatic, is to put it in 3rd and drive at 70, and the RPMs divided by 1000 will match the gear ratio. IE with the standard A4 rear end, in 3rd gear at 70 the RPMs will read just over 2700 RPMs. Can't speak to the manuals personally, but I do believe the same is true but you have to be in 4th at 70.
 
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