Interesting comment. Shorter rear end gears will absolutely have an effect on the RPM drop between gears, as the overall gear ratios in each gear are closer together. I wouldn't disagree that either a 4.10, a 4.56, or a 4.88 gear would really wake up the car, but I've read a gear manufacturer cannot go shorter than a 3.90 without going to a smaller pinion gear, which will adversely effect rear gear life. I would be happy to see a 3.90 made available.

 

To get the RPM drop between gears, you don't even need to know the rear end ratio...I stand by my comment that "Different rear end gears will not do anything for the RPM drop between gears...".

Example with the Z51 M7 with gear ratios of 2.97, 2.07, 1.43, 1.0, .71, .57, and .48 for 1st-7th and using a 6500 RPM shift point:

RPM drop from 1st-2nd: 6500/2.97=2189 RPM output shaft speed. 2189*2.07=4530 RPM engine speed after shift for a 1970 RPM drop between gears. Notice how I did the calculation without the rear gear ratio??? Proof it doesn't have an affect on the RPM drop.

RPM drop from 1st-2nd: 1970 RPM
RPM drop from 2nd-3rd: 2010 RPM
RPM drop from 3rd-4th: 1955 RPM
RPM drop from 4th-5th: 1885 RPM
RPM drop from 5th-6th: 1282 RPM
RPM drop from 6th-7th: 1026 RPM but who cares, you'll never see that shift with 3.42 rear gears. In fact, you'll never see a 5th gear redline shift because you'd have to be going 212 MPH.

For the Z06 M7 with gear ratios of 2.29, 1.61, 1.21, 1.0, .82, .68, and .45 using the same 6500 RPM shift point:
RPM drop from 1st-2nd: 1930 RPM
RPM drop from 2nd-3rd: 1615 RPM
RPM drop from 3rd-4th: 1128 RPM
RPM drop from 4th-5th: 1170 RPM
RPM drop from 5th-6th: 1110 RPM
RPM drop from 6th-7th: 2199 RPM again, you'll never see that shift with 3.42 rear gears. If you could get to that shift, it would kill acceleration as the engine would fall too far off the power curve. You would have to be careful picking the rear gear with this transmission since top speed or a standing mile race could be adversely affected.

Speed in 6th at 6500 RPM with:
3.90 rear gears: 195 MPH
4.10 rear gears: 185 MPH
4.56 rear gears: 167 MPH
4.88 rear gears: 156 MPH
Anything other than the 3.90 gear will result in a drop in top speed. The 4.56 and 4.88 gear may affect a standing mile race or road racing on certain tracks with a long enough straight. The 4.88 would be the best for 1/4 mile racing but you'll have to make 4 shifts to cross the line at the top of 5th...on a positive note, it'll increase average HP during the run substantially making it worth the extra shift. In addition, the 2.29*4.88 combo is better than the 2.97*3.42 combo off the line.

The bottom line is know your race and choose the appropriate weapon.

 

I agree with your logic. No argument here.

But you fail to mention that the higher numerical rear gear will allow you to make up the drop in RPM quicker.

And thus it will help.

 

Are you saying the higher numerical rear gear allows better torque multiplication to make up the RPM drop?

 

No I'm saying that the higher numerical rear gear causes the RPM to increase at a faster rate, hence you get back to the desired Rpm quicker. So the RPM drop between gears has less of a effect. Not eliminated but less.

It's the same reason why a higher numerical rear gear is better than a low one in a car, everything else being the same.

 

Glass is saying the same thing just the technical answer for it. The rear gear change changes the torque mulitplication, if you were to just use a calculator to go through the gearing starting with your engine torque that would be the true torque at the rear wheels. Example 400 ft/lbs of torque through a 3.42 gear would roughly be 1368 ft/lbs, but tire size also has a play in this. Now take the same car with a 4.10 gear and you have 1640 lbs of torque. So with more torque the car will accelerate faster. As he pointed out though the RPM drop is still the same. You can't use this method to pick your gear though, if you numerically when up the torque would keep going up, but the speed you be going down. Ideally you want to be just before redline at the traps so pick your gear that is going to give you that.
by the way before anyone wants to get real technical, this example was on 4th gear with a 1:1 ratio. Obviously the transmission gear ratios will also change the torque multiplication.

 

I have no problem with that advise. I have been drag racing for 50 years and that is how I pick my rear gear.

What I don't understand is why you are so hung up on being so technical. The bottom line is a higher numerical gear will improve the acceleration of a vehicle.

 

Yankee, I'm not about being technical at all, matter of fact the simpler the explanation the better most people understand it. That wasn't for you, it was to end this conversation so that some others would not start spewing technical answers when what you were saying was the same thing and correct.

 

Looks like we are on the same page on this J.

Thanks for the clarification.

 

I would like to try some 3.90s . In my c7 6A

 

Could the M7 stock 3.42s be installed in a C7 A6 or A8? And if so, would acceleration performance, either from a stop or from a highway roll, be increased?

 

3.90 might not be bad in manual but not auto. Will make 1st and 2nd too tight. End up spinning...
Think anything tighter than 3.90 in manual will make the car only good at drag racing with drag radials

I have decent amount of power and looking to do the Z06 manual gear ratios. Mainly because the wider gears I stay in 3rd much longer at drag strip and road courses 2nd be more useful . When I do to speed races that 3-4 shift won't kill me and then the 5th gear


For most race tracks I think Z51 great . I stay in 3rd most the time.

 

I find it very surprising that no one has done a rear end gear swap for the C7 yet.
I own a 2104 C7 A6 Z51. It has the 2.73 gears in it. I would think that the manual 3.42 gears would fit in the same pumpkin. Does anyone know this for sure?
Does anyone know if I can just buy the 3.42 gear set from GM and do a clean swap?


Those 3.42 gears in the A6 Z52 would really wake that car up!


Base C7 manual rear ratio: 3.42
Base C7 auto rear ratio: 2.56

Z51 C7 manual rear ratio: 3.42
Z51 C7 auto rear ratio: 2.73

 

Gears tight in auto.. Just add power.. Heartbeat maggie 😍

 

A 3.42 would really make these a6 cars a beast!!!

 

wonder how much 3.73 or 3.90 gears would help on a cammed ( stage 3 tsp ) car going into high 10's already on stock 2.56 gears with 3000 stall converter ?

 

If A8 first and second gear would be extremely short. Great question !!

 

There are some threads in 3:90 gears. I have a set in my GS M7...I absolutely love them. How the car should have come.