Putting the finishing touches on my build and it's time to decide what to do with the rear end.

I have a DSS driveshaft and FRPP 624HP supercharger kit, suspension, watts link, etc.... The goal is to eventually be at 600WHP for this car.

Current Daily Driving wheels / tires: 18x9.5 with 285/35 R18 Pilot Super Sports

Current Track Tires wheels: 18x10 with 295/30 R18 Toyo R888

In both sets I will want a slightly taller sidewall next time around, especially the daily drivers.

I do HPDEs and will be doing some autocross, I have never been to the drag strip and don't anticipate it being a regular thing, open tracking / corner carving is what I enjoy most.

I already have gear whine from my factory installed 3.73 gears on my 13 GT that is annoying as hell on the freeway.

With the supercharger 1st and 2nd are just about worthless.

Decision 1: What gear ratio to go to? Ford Racing now offers 3.31 and 3.55 as well as some others. Trying to decide between 3.31 and 3.55, thoughts? Leaning towards the 3.31 since the supercharger generates so much torque, and I am fighting against the small diameter on the tires as well, going all the way down that low shouldn't hurt me too much on the track.

I'll get better fuel economy and be able to put the power down better, any reason I should look harder at the 3.55 gears?

Decision 2: As long as I am having the gears changed, going to upgrade the diff at the same time. Just about decided on the Wavetrac. I had a Quaife on a previous car and loved it. The Wavetrac looks very nice and the lifetime warranty is great compared to the T2. Don't want to be re-building the factory T-loc all the time. But could use a sanity check on this.

Thanks!

 

Wait till after its on the dyno. If you're close to 600 at low rpm then 373 will be a blur in 1st and 2nd. 355 is a good gear for that kind of power if you're not primarily drag racing it.

Also I would really like to know what you think about the Toyo R888's. especially how well it does launching on them. I want them but not sure how well they hook.

 

Having driven it for a little while with the 3.73 (which came with the car) already, I have a good for feel for 1st and 2nd being pretty well worthless now. Hence the desire to go down to 3.55 or 3.31.

As for the Toyo's haven't run a track day with them yet on the new set-up but ran them previously. Didn't launch them hard, but really enjoyed them around the track. Lots of grip in the corners and nice consistent transitions.

 

331,s with your set up!!!Just my 2 cents.

 

What class do you want to run in autocross?

3.31 will give you that extra oomph in 2nd and may keep you from having to shift to 3rd

 

Because of the modifications (Supercharger and others) I'll be running in a class I won't be super competitive in, it's more for fun for me. Classwise I would be running in SM (Street Modified)

Sounding like I should be leaning towards the 3.31 gears for road course, auto-x work. The 3.73 were great when it was N/A but with the supercharger on there I need more room to run in each gear.

 

Found this calculator .... Pretty handy.

http://www.catherineandken.co.uk/sti/tyres.html

With my tire sizes, it really lends itself to showing that I should be going with the 3.31 or even newly available 3.27, or even 3.15 (but I think the ideal might be 3.27, especially if for the next set of tires I can go back to a 40 series sidewall). I don't have a shortage of torque to worry about getting the car moving forward quickly with the longer 1st and 2nd.

Looked at this handy calculator as well to see what RPM I would be at for certain speeds.

http://www.allfordmustangs.com/Detailed/608.shtml

 

I think the 13 Shelby's are running 3.31 and putting down 600rwhp

 

They are running 3.31 .. BUT .... the transmission in the Shelby has different ratios so it's an apples to oranges comparison ....

The GT500 is: 2.66, 1.82, 1.30, 1, .77, .50

The GT is: 3.66, 2.43, 1.69, 1.32, 1.0, .65

So what a 3.31 does in a GT is very different from what it will do in a GT500.

IE ... with my tire size ... a 3.31 on the GT 500 gearing gets me to 61 MPH in 1st gear

on the GT transmission it gets me to 44 MPH in 1st gear

You'll never quite get a GT to behave like a GT500 transmission, but to get them close you would need a rear end ratio < 3.

This is why I am looking at the 3.27 and 3.15 with the most seriousness for consideration right now.

Found this ... Interesting:

2011 Mustang 5.0 w/Tune - redline @ 7200
3.08 @ 7200 rpms
1 = 51 mph
2 = 77 mph
3 = 110 mph
4 = 141 mph
5 = 187 mph
6 @ 80mph = 2000 rpm

2011 Shelby GT500
3.55 @ 6250 rpms
1 = 47 mph
2 = 79 mph
3 = 108 mph
4 = 141 mph
5 = 188 mph
6 @ 80mph = 1775 rpm

 

Jared, gears are - by definition - a torque multiplier as you know. And you supercharger provides more than enough torque already. Going from 3.73's to 3.55's is just not enough difference to make a difference.

3.27's were standard Mustang GT gears a decade ago and the 3.31's came out in 2005. Both are essentially the same ratio, so either will feel exactly the same. These might be the right answer for your combination, but you may also want to consider 3.08 or 3.15 gears since you have so much torque on tap and aren't going to drag race much, if at all.

 

Yup ... After lots of reading, searching, and more reading and searching.

I have come to the conclusion that my choice with my available torque, suspension mods, and tire size should really be between the 3.27, 3.15, and 3.08.

It's been hard to find decent feedback from folks that have had those sizes installed though. Lots of 3.31, 3.55, and 3.73 feedback.

Really on the fence, almost leaning towards the 3.15 over the 3.27 ... but part of me still wants to consider the 3.08. The 3.08 would make the gearing pretty similar to a 2011/2012 GT 500 with a 3.55.

 

3.08's may very well be the hot ticket for your combo, considering you have GT500 levels of power with GT gear spacing in the transmission.

On a side note, I have personally owned Mustangs with the following gear ratios over the years: 2.73, 3.27, 3.31, 3.55, 3.73, 4.10, 4.30, and 4.56. Most were naturally aspirated, one was with a supercharged GT. Gears are a very critical piece of any combination of mods, so choose wisely.

 

I always thought the gears were going to be important but wanted to give it a go with the factory installed 3.73s before I went for the change.

Knowing how critical they are, is what makes the choice so tough. Ha ha. I only want to do it once.

 

Finally had the 3.08s installed.

Just doing the break-in, but so far I think I'll be very happy with them. Traction will be found where it was lacking before, no more gear whine, Highway cruising is at a reasonable RPM, and I should be able to get on it coming out of corners on the road course now in combination with the wavetrac differential I had installed at the same time.

Going with and sticking with the 18" tire size is what made the final decision to go all the way to 3.08. Initial impressions are favorable and so far happy with the decision.

Will get them a real workout at the Shelby Bash in a couple of weeks.

 

I think you made a good choice. And your highway mileage is going to be amazing!