Richard, the rear axle ratio of our cars is 2.56 to 1. Replacing it with a 3.42 would be a 33% increase of rpm per mile. Since speed is linear it would result in a 1/3 reduction in top end, since our top end is limited by the engine management computer, it is hard to tell, but it would yield probably a top end of somewhere near 110 -115 mph. Further our gas mileage would reduce by about 1/3 or be somewhere in the neighborhood of 10 mpg on the highway.
The car would be very fast in the quarter mile and probably red line at the stripe in 5th. But I wouldn't want to drive it on the street.
The corvette probably has a stock ratio in the low 3's, or thereabouts, so the jump to 3.45 isn't so great, yielding the results the article proclaims.
Just be careful to research actual results with our cars, before making such a costly mod.
Just my 2c
Rusty
It looks like you have 4 concerns:
- Reduction in fuel economy
- Limited top end speed
- Drivability
- Cost
I did a significant amount of the research on this - you are technically accurate, but the real world data is not consistent with the math.
** Reduction in fuel economy
First off - gas mileage is not as impacted as you'd think... this was my first assumption as well - great minds think a like.
But what I've learned is that since the gearing is lower, there is less load on the engine... In that, with the taller gears it becomes easier to turn the input on the diff... so yes, the RPM is higher, but it uses less fuel to do it... real world expereince from drivers who have 3.42 gears in their A6 vettes all report no change in MPG or slightly improved MPG. Less load on motor equals less fuel used. (which is why when you put the car in neutral and run the engine at 2k rpm it uses less fuel than when you are driving and have the car at 2k rpm with 2 people and a trunk full of luggage... same RPM but different loads on the engine)
This is also why your MPG goes down when you put larger rolling-diameter tires on a car... the drivetrain load increases bringing MPG down despite the fact that RPM went down... put on smaller rolling-diameter tires on and your MPG will increase - despite the face that the RPM goes up...
I agree that its a bit counter-intuitive but thats whats happen in real-world.
** Limited top end speed
The A6 c6 and the XLR have the same A6 transmission... the 4L80e.
The A6 C6 cars also have the same diff as the XLR with 2.56 gears (the C6 Z51 also has an uncommoon 2.73 gear option).
The XLR-V and the A6 C6 cars do NOT have the same torque converter. The A6 has an 11inch converter with a single plate TCC... the XLR-V has a 10.5" with a dual-plate TCC. The translates into a different stall rpm and a more durable TCC so it can handle more tq - which makes sense since the XLR-V is blown and has a flat tq curve and the typical A6 C6 is naturally aspiraed with a typical sweeping tq curve.
The vette M6 and the vette A6 do not have the same rear end gearing nor the same gear ratios between shifted gears... additionally the A4 and the A6 do not have the same gearing between shifted gears - they are as follows:
The effective gearing ratio with the various diff gears is the following:
When doing the math, you will see what the RPM speed is at various speeds with stock gears on the A6:
Code:
RPM 1st 2nd 3rd 4th 5th 6th
500 3.80 6.47 9.97 13.27 17.95 22.77
600 4.55 7.76 11.97 15.92 21.54 27.33
700 5.31 9.05 13.96 18.58 25.13 31.88
800 6.07 10.34 15.96 21.23 28.72 36.44
900 6.83 11.64 17.95 23.88 32.31 40.99
1000 7.59 12.93 19.95 26.54 35.90 45.55
1100 8.35 14.22 21.94 29.19 39.49 50.10
1200 9.11 15.52 23.94 31.84 43.08 54.66
1300 9.87 16.81 25.93 34.50 46.67 59.21
1400 10.63 18.10 27.92 37.15 50.26 63.77
1500 11.39 19.40 29.92 39.81 53.85 68.32
1600 12.15 20.69 31.91 42.46 57.44 72.88
1700 12.91 21.98 33.91 45.11 61.04 77.43
1800 13.66 23.28 35.90 47.77 64.63 81.99
1900 14.42 24.57 37.90 50.42 68.22 86.54
2000 15.18 25.86 39.89 53.07 71.81 91.10
2100 15.94 27.16 41.89 55.73 75.40 95.65
2200 16.70 28.45 43.88 58.38 78.99 100.21
2300 17.46 29.74 45.88 61.04 82.58 104.76
2400 18.22 31.03 47.87 63.69 86.17 109.32
2500 18.98 32.33 49.87 66.34 89.76 113.87
2600 19.74 33.62 51.86 69.00 93.35 118.43
2700 20.50 34.91 53.85 71.65 96.94 122.98
2800 21.26 36.21 55.85 74.30 100.53 127.54
2900 22.02 37.50 57.84 76.96 104.12 132.09
3000 22.77 38.79 59.84 79.61 107.71 136.65
3100 23.53 40.09 61.83 82.26 111.30 141.20
3200 24.29 41.38 63.83 84.92 114.89 145.76
3300 25.05 42.67 65.82 87.57 118.48 150.31
3400 25.81 43.97 67.82 90.23 122.07 154.87
3500 26.57 45.26 69.81 92.88 125.66 159.42
3600 27.33 46.55 71.81 95.53 129.25 163.98
3700 28.09 47.85 73.80 98.19 132.84 168.53
3800 28.85 49.14 75.80 100.84 136.43 173.08
3900 29.61 50.43 77.79 103.49 140.02 177.64
4000 30.37 51.72 79.78 106.15 143.61 182.19
4100 31.12 53.02 81.78 108.80 147.20 186.75
4200 31.88 54.31 83.77 111.46 150.79 191.30
4300 32.64 55.60 85.77 114.11 154.38 195.86
4400 33.40 56.90 87.76 116.76 157.97 200.41
4500 34.16 58.19 89.76 119.42 161.56 204.97
4600 34.92 59.48 91.75 122.07 165.15 209.52
4700 35.68 60.78 93.75 124.72 168.74 214.08
4800 36.44 62.07 95.74 127.38 172.33 218.63
4900 37.20 63.36 97.74 130.03 175.92 223.19
5000 37.96 64.66 99.73 132.69 179.52 227.74
5100 38.72 65.95 101.73 135.34 183.11 232.30
5200 39.48 67.24 103.72 137.99 186.70 236.85
5300 40.23 68.54 105.71 140.65 190.29 241.41
5400 40.99 69.83 107.71 143.30 193.88 245.96
5500 41.75 71.12 109.70 145.95 197.47 250.52
5600 42.51 72.41 111.70 148.61 201.06 255.07
5700 43.27 73.71 113.69 151.26 204.65 259.63
5800 44.03 75.00 115.69 153.91 208.24 264.18
5900 44.79 76.29 117.68 156.57 211.83 268.74
6000 45.55 77.59 119.68 159.22 215.42 273.29
6100 46.31 78.88 121.67 161.88 219.01 277.85
6200 47.07 80.17 123.67 164.53 222.60 282.40
6300 47.83 81.47 125.66 167.18 226.19 286.96
6400 48.59 82.76 127.66 169.84 229.78 291.51
6500 49.34 84.05 129.65 172.49 233.37 296.07
6600 50.10 85.35 131.64 175.14 236.96 300.62
6700 50.86 86.64 133.64 177.80 240.55 305.18
6800 51.62 87.93 135.63 180.45 244.14 309.73
6900 52.38 89.23 137.63 183.11 247.73 314.29
7000 53.14 90.52 139.62 185.76 251.32 318.84
7100 53.90 91.81 141.62 188.41 254.91 323.40
7200 54.66 93.10 143.61 191.07 258.50 327.95
7300 55.42 94.40 145.61 193.72 262.09 332.50
7400 56.18 95.69 147.60 196.37 265.68 337.06
7500 56.94 96.98 149.60 199.03 269.27 341.61
And here they are with 3.42 gears:
Code:
RPM 1st 2nd 3rd 4th 5th 6th
500 2.84 4.84 7.47 9.93 13.44 17.05
600 3.41 5.81 8.96 11.92 16.12 20.46
700 3.98 6.78 10.45 13.90 18.81 23.87
800 4.55 7.74 11.94 15.89 21.50 27.28
900 5.11 8.71 13.44 17.88 24.19 30.69
1000 5.68 9.68 14.93 19.86 26.87 34.09
1100 6.25 10.65 16.42 21.85 29.56 37.50
1200 6.82 11.62 17.92 23.84 32.25 40.91
1300 7.39 12.58 19.41 25.82 34.94 44.32
1400 7.96 13.55 20.90 27.81 37.62 47.73
1500 8.52 14.52 22.40 29.80 40.31 51.14
1600 9.09 15.49 23.89 31.78 43.00 54.55
1700 9.66 16.46 25.38 33.77 45.69 57.96
1800 10.23 17.42 26.87 35.76 48.37 61.37
1900 10.80 18.39 28.37 37.74 51.06 64.78
2000 11.36 19.36 29.86 39.73 53.75 68.19
2100 11.93 20.33 31.35 41.71 56.44 71.60
2200 12.50 21.29 32.85 43.70 59.12 75.01
2300 13.07 22.26 34.34 45.69 61.81 78.42
2400 13.64 23.23 35.83 47.67 64.50 81.83
2500 14.21 24.20 37.33 49.66 67.19 85.24
2600 14.77 25.17 38.82 51.65 69.87 88.65
2700 15.34 26.13 40.31 53.63 72.56 92.06
2800 15.91 27.10 41.81 55.62 75.25 95.47
2900 16.48 28.07 43.30 57.61 77.94 98.88
3000 17.05 29.04 44.79 59.59 80.62 102.28
3100 17.62 30.01 46.28 61.58 83.31 105.69
3200 18.18 30.97 47.78 63.56 86.00 109.10
3300 18.75 31.94 49.27 65.55 88.69 112.51
3400 19.32 32.91 50.76 67.54 91.37 115.92
3500 19.89 33.88 52.26 69.52 94.06 119.33
3600 20.46 34.85 53.75 71.51 96.75 122.74
3700 21.03 35.81 55.24 73.50 99.44 126.15
3800 21.59 36.78 56.74 75.48 102.12 129.56
3900 22.16 37.75 58.23 77.47 104.81 132.97
4000 22.73 38.72 59.72 79.46 107.50 136.38
4100 23.30 39.69 61.21 81.44 110.19 139.79
4200 23.87 40.65 62.71 83.43 112.87 143.20
4300 24.43 41.62 64.20 85.42 115.56 146.61
4400 25.00 42.59 65.69 87.40 118.25 150.02
4500 25.57 43.56 67.19 89.39 120.94 153.43
4600 26.14 44.53 68.68 91.37 123.62 156.84
4700 26.71 45.49 70.17 93.36 126.31 160.25
4800 27.28 46.46 71.67 95.35 129.00 163.66
4900 27.84 47.43 73.16 97.33 131.69 167.06
5000 28.41 48.40 74.65 99.32 134.37 170.47
5100 28.98 49.37 76.15 101.31 137.06 173.88
5200 29.55 50.33 77.64 103.29 139.75 177.29
5300 30.12 51.30 79.13 105.28 142.44 180.70
5400 30.69 52.27 80.62 107.27 145.12 184.11
5500 31.25 53.24 82.12 109.25 147.81 187.52
5600 31.82 54.21 83.61 111.24 150.50 190.93
5700 32.39 55.17 85.10 113.22 153.19 194.34
5800 32.96 56.14 86.60 115.21 155.87 197.75
5900 33.53 57.11 88.09 117.20 158.56 201.16
6000 34.09 58.08 89.58 119.18 161.25 204.57
6100 34.66 59.04 91.08 121.17 163.94 207.98
6200 35.23 60.01 92.57 123.16 166.62 211.39
6300 35.80 60.98 94.06 125.14 169.31 214.80
6400 36.37 61.95 95.55 127.13 172.00 218.21
6500 36.94 62.92 97.05 129.12 174.69 221.62
6600 37.50 63.88 98.54 131.10 177.37 225.03
6700 38.07 64.85 100.03 133.09 180.06 228.44
6800 38.64 65.82 101.53 135.08 182.75 231.85
6900 39.21 66.79 103.02 137.06 185.44 235.25
7000 39.78 67.76 104.51 139.05 188.12 238.66
7100 40.35 68.72 106.01 141.03 190.81 242.07
7200 40.91 69.69 107.50 143.02 193.50 245.48
7300 41.48 70.66 108.99 145.01 196.19 248.89
7400 42.05 71.63 110.49 146.99 198.87 252.30
7500 42.62 72.60 111.98 148.98 201.56 255.71
As you can see - given enough power, the A6 trans with 3.42 gears an easily go north of 200mph.
As for the ECM limiting speed - my XLR-V had that removed years ago... but once the TCM and ECM are recalibrated with the diff gear swap (which you is required) the computers will have no problem with the new gears, even if you still have the 155mph limiter in place.
** Drivability
From everything I've researched - driveability hasn't changed much... in fact, most people (if not all) did not even notice a difference going from the 2.53 gears to the 3.15... and everyone suggests not bothering with a 2.53 -> 3.15 swap and just going directly to a 3.42...
Everyone describes the 3.42 swap as incredibly fun to drive... but really insignificant at the track - since going to a 4.10 or higher gear is really where you get significant improved #'s at the track.
So if doing a 3.15 swap yields not much of a difference, I can only believe that the 3.42 will also not change the driving characteristic much.
** Cost
People are also reporting lower failure rates with a diff-swap vs any other performance improvements. Mainly do to the fact that:
- It lowers engine load so the engine, torque converter, and transmission are all under less stress
- Its an incredibly simple modification - its just a gear swap... no high boosting the engine, no larger fuel injectors, etc...
Additionally, I can't think of anything else that is really less than a couple of grand in terms of cost when doing a major upgrade... Even the Corsa cat-back exhaust is about $1500 - and the dyno proved that I didn't get much for it... The Stage 2 intake kit from D3 also isn't cheap - let alone their one-thousand dollar pulley swap they have... Hell, I spent more on new tires than what this gear swap will cost (buying and mounting Michelin PS2's aren't cheap
)
So in relative terms this upgrade isn't all that expensive.
So you get a quick power upgrade with no additional stress on your engine. Sounds like a win-win to me.
I'm open to hearing any more feedback - but all signals point to GO on this... but I'd love to hear something to the contrary - I'd hate to get a case of buyers remorse after the fact.
