What Gear? Current 3.31
 

2013 Mustang GT
Vortech Centrifugal Supercharged
Boss 302 Manifold
Lowered suspension about 1 to 1.5 inches and tight
Stock Manual Transmission
Mcleod RXT Clutch
Nitto 555 Summer 275/35r20 tires

RWHP 670
Torque 520

Drives more like a stock car. Great on the highway.

Can I go with a 3.73 gear?
Can I still use First gear if I do? lol
How much will it affect my top speeds and shifts?
I can get to 60 in 2nd gear, but that last few miles per hour is close to red line.

I have gotten use to the acceleration and since my car does not start boosting till around 3000 RPM I am thinking a shorter gear.

Just got the clutch. Teared up the stock clutch a few months ago. It really grabs now with the RXT.

Pros and Cons with my setup with 3.73 or 3.55 gears?
How would it drive?

Also with my rims or new rims might eventually go 315/35/r20 tires if the gear is too short for my current tires.

Running a stage 2 procharger @9psi. Not sure of hp as I have not been on a dyno yet. My car has 3.73s now and 1st and 2nd gears are absolutely usless as soon as boost hits. Also spins at the top of third a little. Definitely looking to go to a 3.55 or 3.31 gear this year.

The 20" wheels are "hurting" your gear, come down to a 19" wheel for 3.73:1 or, do 3.55:1 as this is why I only like 18" wheels.

It's probably safe to assume that you're having grip issues in anything below 4th gear, so by putting in a more aggressive final drive you won't see any better acceleration in those gears, and will probably just spin even more. The advantage in acceleration will come in the higher gears (4-6). Don't worry about if you shift into 3rd before 60 as it is a pretty arbitrary number, unless you are doing testing for MotorTrend. The more important number is the trap speed in the quarter mile, which you will more than likely be in 4th for either the 3.31 or 3.73 gears. I've included a link to a gear ratio calculator to play around with.

As far as putting a shorter gear in just to put on a larger diameter wheel, you would be spending money to spend money, as they would more or less counteract each other. There can be an argument to be made if you plan to spend a lot of time at the drag strip though.


http://www.dragtimes.com/2011-Ford-M...tton=Calculate

Is there a big difference between 3.31 and 3.55?

Maybe cause of the power. The owner of the shop runs the same setup without the boss manifold at 650hp and has 3.73. I am about 750+HP with the manifold, but that is mostly at higher RPM.

The car seems very controllable. Maybe too controllable. Lol

Thanks for the reply all.


Well I was gonna go 3.73. And if it was too much then go slightly bigger tire as a fine tune to turn it down some with more grip and diameter.

Also the bigger diameter tire compensates for top end speed with a shorter gear.
However wider rears will affect the turning some I gather?

Yes, these Mustang's have a habit of understeering at the limit when the rears are wider than the front. Unless you use the car for Autocross or road coarse duty, then you probably won't notice much of a difference since you shouldn't be going 10/10th's on public roads anyway.

What Gear? Current 3.31
 

Fireball241, I'll tell you a little secret. Forget about the rear end gears. 3.31is close enough. The gears that must be changed are the gears in your TRANNY. Back in the day I learned this secret from "oval racing." Check with the fellas at "Pro Motion Performance." Not to get to crazy, but here is a sampling of what I am talking about. Also, this is what I had to do to get excellent 60 footers and beautiful launches into the Twilight Zone. To determine any car’s overall First-gear ratio, multiply the transmission’s First-gear ratio by its rear-gear ratio. If your Mustang has a 3.35 First gear and a 3.55 rear gear, the overall First-gear ratio is 3.35 x 3.55 = 11.89. Anything greater than 9.5 could be considered overkill for the street. Ever driven a Mustang with a 3.35 First-gear trans and a 3.73 rear? First gear is virtually useless, since the overall ratio is 12.49. This is why a 2.95 First-gear T5 or the Tremec 600 2.87 trans is far more desirable if your plans include deep rear gears.

Great stuff. I am curious about the comparison car mentioned above and the transmission it has.

However a 3.73 may expedite the upgrade of my stock transmission especially with bigger tires ;)

What Gear? Current 3.31
 

Fireball, you are scaring me. Why, because you sound a lot like my thinking. Yes, the "taller" tires will cut down your rear gear ratio, but this will give you less "rollout" and effect your ET. Don't change your tire diameter before you change any gears. Tire size can always be adjusted. If you make "Transmission" gear changes, you just might get by without spending money on changing your "rear ends" gear ratio. Gearing and tire size has always been a "dark art." I changed the T-5 "tranny" gear set in my 1987 Foxbody with a 331 stroker, and it worked out perfect. Yup, I started out with 373s and worked my way down to 325s, and that was the right combination for "this" car. I am running NITTO NT555R P275/40R17, street/strip presently. Come summer, I am mounting Mickey Thompson Radials on 15" rims, strictly for competition. Gotta love the bigger sidewall. Take your time, and success will come to you. It only took me 60 years to get racing down to a fine point. By the way, you outline a very nice set up in your first post. If you leave everything the same, I would vote for 355 rear end gears, and have some fun.