bakerjd

Ok guys, I know there are 5 million gear threads but I've read through numerous threads and can't seem to find a definate answer to the question I have. I want to know WHY people say that 3.73 gears are better than 4.10 gears for an auto if you plan on getting a supercharger! I mean what is the factual, science based evidence that sets this 'rule' in stone!? My girlfriend has decided to buy me the gears I have been wanting for christmas and I want to do it right the first time! I plan on eventually rebuilding the engine and tranny and throwing a Kenne Bell Mammoth on there and I'm pretty certain i'd probably need to upgrade the rear end as well at that time (I know this will run me around 15k eventually so i'm glad i'm good with finances, lol). This is going to happen WAY down the road so i'm thinking 4.10's would be the best for right now because of the auto and NA engine setup. Let me know what you think and any information is welcome. This is my DD and i'm not really worried about gas mileage, I mean, I did buy a Mustang GT! lol

2005Redfire6

They may be telling you that because of the power you will be putting out with a supercharger, therefor the 4.10s can lead to wheel spin. I guess thats their logic? But... With a centrifugal (not sure which one you would go with) the 4.10s are great, gets the blower into the RPM's faster (obviously).

bakerjd

I'm going to Twin screw it! I have heard great things about the power throughout the band with twin screw sc's, not to mention kenne bell's are friggin schmexy! lol

2005Redfire6

Oh hell yeah they are! On that note you may be better off with 373s.... Trust me, the low end torque on those things and 4.10s would be realllly quick but you will probably have serious traction issues.

bakerjd

I should probably mention that the kenne bell won't be on while this vehicle is a daily driver, lol...right now it is a daily but that will only last until the payments are cleared and I purchase another GT, haha

onederful100

there is no rule for gears and a SC on an auto or MT.
these rules that many speak of are not set in stone.
gears and any mod should be considered with the car as a whole, and how it functions for what you want it for.

there is also the tranny gearing to consider along with the rear end gearing.
i'm not sure exactly, but the auto may have a lower 1st gear ratio than the MT. and also the 5th gear ratio might be different in the auto, but i'm not sure exactly.
there are other things that effect your accelleration besides the rear end gears.

seeing you have a 06 GT, you probobly have 3.31's
3.73's will make a difference in comparison with your current gearing. and 4.10's even moreso.
but the only real way to find out what is best for you is to drive a auto Mustang with 3.73's and 4.10's and see what you like better.

but gearing and SC with auto is not set in stone.
i have 4.10's, Whipple SC on a manual tranny. most say that it is too much.
but i change my driving a little and i can handle it just fine.
i also have taller tires, 28.5", and that helps decrease the 4.10 overall effect.

2005Redfire6

+1 to the rear tires making a difference as well. I have a 295/45/18 which comes out to a 28.5 tire I believe.

JIM5.0

The bottom line is hook-up! If you have all the torque in the world and the deepest gear ratio possible, it is all nothing because you will be redlining and going nowhere. The classic spinning your wheels for nothing.

The thing to consider here is what kind of tires you have, how well they grip, and what your first gear ratio is in your tranny.

Lets look at gear ratio: You must get over rear gear ratio only and look at TOTAL gear ratio. You do this by multiplying your 1st gear ratio with your rear gear ratio.
Lets use the MT82 1st gear ratio vs the TR6060 1st gear ratio.
The TR6060 1st gear I am talking about is 2.97:1. (You can get a TR6060 with a shallower first, but we will look at the 2.97:1 here).
The MT82 has a much deep[er first, 3.66:1.

Now, lets consider TOTAL gear ratio:
The stock MT82 comes with a 3.31:1 rear gear. The total gear ratio is: 12.11:1
Take the TR6060 with its 2.97:1 1st and slap on 4.10:1 rear gears and you get almost the same thing! 12.18:1!!!!

On the MT82, the 4.10:1 rear gear would give you probably too much total gear ratio: 15.06:1.

So, just by looking at transmissions, on some you actually DO want 4.10:1 rear gears. But if you have a tranny with a deeper 1st gear, going 4.10:1 rear gears could be too much.

Now onto tires: If you have street tires and gobs of torque, going too deep of a total gear ratio is nothing but spinning wheels. Shallower gear ratios help mitigate that.
But if you have drag radial slicks, and they are some damn tall and wide tires, you will want as deep a total gear ratio as possible because those tires will require well above 1000 ft-lbf at the wheel to slip.
On the MT82, 4.10:1 rear

bakerjd

Wow guys, this is a lot more complicated than I first thought. Thanks onederful and Jim, I really appreciate you helping me to understand this more thoroughly. I won't be swapping the tranny anytime soon so i'm thinking i'm going to go with the 4.10s and rethink things once I rebuild the powertrain later on. I just wanted reassurance that I was going about this the right way and it seems like an educated purchase. Thank you so much for clearing this up for me.

side note, i'm running 255/45-18 and 275/40-18 NITTO Extreme Performance NT555 Tires on my saleens and will be getting blizzaks on my stock rims once the tread gets low on the current setup. When I rebuild the powertrain I will probably throw some MT street radials for shows and strip!

Alec

Stop being a pansy and get a 4.56