Differential gear options

[Arizona Vipers]

What are our options on gears? Is it just 3.07, 3.21 and 3.55?

[Dan Cragin]

2.73 3.07 3.21 3.33 (hard to find) 3.45 3.55 3.73 3.91 4.10 and 4.56

[Arizona Vipers]

Thanks Dan! So they don't make 3.21 or 3.33 anymore? I have to find old stock?

[nwa_viper]

I have the 3.91s in my '16 SRT. It was like that when I bought it and have never driven one with 3.55s but I turn around 2300 rpms at 70mph for comparison.

[Dan Cragin]

The 3.21's are available. The 3.33's are hard to find as they are no longer made.

Dan

[serpent]

Has anyone swapped to 3.73 gears? Curious to know if it may or may not improve the car.
Acceleration on the street, road course use, drag racing and how was the fuel economy?

[Arizona Vipers]

In my quest for finding the perfect diff gear for my '13 track car, here is some useful info I've calclulated for everyone here, street guys, drag racing guys and road racers alike.
Here are the maximum speeds in each of our 6 gears with all the different differential gears available for the Gen 5. These calculations are based on the factory rear tire size of 27.4" (355-30-19)
NOTE: 6th gear speeds listed are obviously theoretical if our cars had millions of horsepower. Depending on the diff gears, some cars would hit top speed in 5th, some in 6th.
All data is using the factory 6400 RPM redline. If you have Arrow computer or custom tune at 6600 redline, your speeds would be 3.1% higher in each gear.

1st 2.26
2nd 1.58
3rd 1.19
4th 1.00
5th. .77
6th .63


STOCK GEARS 3.55 (miles per hour)
1st 65
2nd 93
3rd 123
4th 147
5th 191
6th 233



DIFF RATIO: 2.73 (miles per hour)
1st 85
2nd 121
3rd 161
4th 191
5th 248
6th 303


DIFF RATIO 3.07 (miles per hour)
1st 75
2nd 108
3rd 143
4th 170
5th 221
6th 270




DIFF RATIO 3.21 (miles per hour)
1st 72
2nd 103
3rd 137
4th 163
5th 211
6th 258




DIFF RATIO 3.33 (miles per hour)
1st 69
2nd 99
3rd 132
4th 157
5th 203
6th 249




DIFF RATIO 3.73 (miles per hour)
1st 62
2nd 89
3rd 118
4th 140
5th 182
6th 222



DIFF RATIO 3.91 (miles per hour)
1st 59
2nd 84
3rd 112
4th 133
5th 173
6th 212




DIFF RATIO 4.10 (miles per hour)
1st 56
2nd 81
3rd 107
4th 127
5th 165
6th 202


DIFF RATIO 4.56 (miles per hour)
1st 51
2nd 72
3rd 96
4th 114
5th 149
6th 182

[Stealth78]

There are two different 6th gear ratios for the GenV, which are you basing this off of?

[Stealth78]

Sorry... Just reread and it sounds like you are basing this off of your 2013 which as far as I know only had the shorter 6th. When did the revised 6th come out, 2015?

[Arizona Vipers]

This is for the .63 6th gear ratio

[Dan Cragin]

3.73 is the most popular Gen 5 rear ratio change. Tends to get a little buzzy with the 3.91's and the cruise control wont work.

[Racingswh]

So based on that, which is great information and thank you, I think for what we do 3.21's or 4.10's are the call.

I am even thinking 4.10's because it turns the thing into a 3 gear car with the lowest gear used being 3rd instead of 2nd with the 3.21's and the 3-4 4-3 shift has to be the simplest of them all. The 2-3 3-2 isn't all that easy that's for sure. I do it all the time but I would rather a 3-4 4-3 shift.

4.56 is too short. Many times I need over 149 mph so I would need to make the shift to 6th and it falls on it's face again only marginally better than putting the thing in 5th now with the 3.55's.

So yep, gear change for me would be the 4.10's.

I am rethinking that again. 5th gear would be needed regularly which is a PITA as well. I need 4th to carry to about 135 mph. 127 is too short. I also need to remember I am using a shorter tire.

I definitely need a change. Just don't know to what yet.

Looking at corner speeds matched to speeds in gear with a 26.8" tall tire like I use the 3.21's look about right. The car would be a 2-3-4 gear car.

[Arizona Vipers]

You run on bigger tracks than I do Racingswh, you would really benefit from a 5th gear change. Stock is .77, you can get .80 or .84. I've got the .80 but want to get the .84.
I'm probably gonna go with the 3.33's if I can find them. If I can't then 3.21 for me. With 710rwhp and 665rwtq this is a better choice for me, i'll be able to do 99 in 2nd gear (103 with the 3.21's). I'll only have to shift on the straights.

[nwa_viper]

3.73 is the most popular Gen 5 rear ratio change. Tends to get a little buzzy with the 3.91's and the cruise control wont work.

“Cruise Control won’t work”??? Why would you think this? I have 3.91s and cruise works fine. I also hit the rev limiter around 128 in 4th not 133. I know because I hit this down the front straight at Heartland Park every lap.

[Racingswh]

“Cruise Control won’t work”??? Why would you think this? I have 3.91s and cruise works fine. I also hit the rev limiter around 128 in 4th not 133. I know because I hit this down the front straight at Heartland Park every lap.

Is the tire height the same? I just used the Tremec gear ratio calculator to check and with a 27.4" tall tire at 6400 RPM it says 133.43.

How is it for road course use? You must have to use 5th a lot? Is the .77 5th gear soft or ok?

Looks like for me based on the numbers the calculator is giving me the 3.73's might work well for me at the tracks I frequent with the .84 5th. It says with the 26.8" tall tire is good for 136 mph in 4th, 162 mph in 5th at 6400 rpm. That's right where I need it to be.

[nwa_viper]

Is the tire height the same? I just used the Tremec gear ratio calculator to check and with a 27.4" tall tire at 6400 RPM it says 133.43.

How is it for road course use? You must have to use 5th a lot? Is the .77 5th gear soft or ok?

Looks like for me based on the numbers the calculator is giving me the 3.73's might work well for me at the tracks I frequent with the .84 5th. It says with the 26.8" tall tire is good for 136 mph in 4th, 162 mph in 5th at 6400 rpm. That's right where I need it to be.

I’m running the factory Pirelli tires front and rear. I never use 5th at the track but haven’t been to any higher speed ones. I just uploaded videos from a track day I ran yesterday: https://www.youtube.com/user/megalifted

[Racingswh]

I’m running the factory Pirelli tires front and rear. I never use 5th at the track but haven’t been to any higher speed ones. I just uploaded videos from a track day I ran yesterday: https://www.youtube.com/user/megalifted

With the Pirelli 355/30/19's At 6400 RPM in 4th with 4.10's the calculator says 127.24. My guess is you're running 4.10's. Not 3.91's.

Plug in your numbers and see what you think.

http://www.tremec.com/calculadora.php

[nwa_viper]

28753
Definitely 3.91s

[Voice of Reason]

If I had the taller 6th like they started doing in 15 I’d go 3.73 or 3.91. The tracks the IL club races at barely get into 4th gear and the idea of making the car purely a 3-4 gear car and eliminating the need for 2nd when racing sounds wonderful.

[Racingswh]

28753
Definitely 3.91s

Since you had me curious because I am usually of the belief that most math is fairly accurate I checked me GPS information and video information against the Tremec calculator and it's very close. Within a MPH or so and sometimes dead spot on when measuring speed in gear at a particular engine RPM.

Point is unless your Pirellis are way shorter than the advertised height or your 4th gear is something other than the stock 1 - 1 ratio then you more than likely have 4.10's or at least 3.91's that think they are.

It's doubtful that it's coincidence that you run out of gear exactly when 4.10's would run out of gear but you actually have 3.91's which mathematically, which I now have verified the calculator I used reasonably, should run out of gear about 133 mph.

A 5 mph variation is doubtful.

Nevertheless it makes no difference. You look like you have an awesome time in the car and it's a beautiful machine no doubt and without question the fastest color!! :)

Keep enjoying it they are amazing cars!!

I get to test my new Giro disc setup in a couple weeks. I am really excited to get back on the track!!

3.73's with a .84 5th is what it will be for my ACR-E in the off season.

[nwa_viper]

Is it possible the calculator does not take into account the height of the tire with the weight of the car on it? Or is the calculator using circumference? Maybe the speedo in my car AND gps on the GoPro are inaccurate. Perhaps they packaged 4.10s in a 3.91 box? I don’t know but I know it won’t do 133 in 4th.

With all that said, I wouldn’t use that calculator to pick gears. I’d ask someone running what I was considering.