# 3.91 Gear Ratio on 06 M6



## firebird (Feb 6, 2011)

i was looking for some input on the 3.91 gearing and and what difference it made. Has anyone here done it, is it worth the money and hassle?


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## jpalamar (Jul 22, 2008)

I rode in a Mustang with 3.31s(stock), mine has 3.55(stock optional), and my buddy has 3.73s(aftermarket). You could def feel the difference. I'm personally going to 4.10s in mine because I'm staying NA and it will be great for the cam grind I plan on doing.


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## svede1212 (Nov 1, 2005)

I have 3.91s on the goat. It makes the car feel a lot lighter and more nimble. With heads and cam it's almost too much in first gear tho. Highway 70mph cruising is 2,200 RPM. Not too bad for mileage.


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## firebird (Feb 6, 2011)

yes i was thinking of 4.10 but was curious if that will be too much or not. last thing i want is to snap the rear end, but just thought of the quickness it would have makes me drool. and i was looking at the prices and there is over 200$ difference between the 3.91 and 4.10

that sounds sweet svede, and not bad at all for the mpg. and at 2.2 rpms. which brand do you have and how much does it cost. i was looking at the gforce and they run about 449$, pricy as heck.


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## jpalamar (Jul 22, 2008)

I would be more worried about snaping the rear from wheelhop then more agreeive gears, power, and sticky tires.


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## svede1212 (Nov 1, 2005)

I think you can go too much. The transmission gearing on these cars already gives a lot of grunt and the 3.91s more than compliments them. I got the Aussie gears in a pumpkin swap with another GTO owner 4 years ago for a steal. He was trapping so fast that he needed to gear down. Once you trap 125-130 it's time to swap back.


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## bayhammer (Apr 10, 2011)

i was told it was necessary to replace the harrop cover when duing a gear swap due to heat....does this sound correct??? or can u just do a gear swap.


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## svede1212 (Nov 1, 2005)

The Harrop mainly helps for rear cradle stability and for heat in extended thrashing like autocross. For "normal" driving it wouldn't be mandatory


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## firebird (Feb 6, 2011)

see i am undecided if i will ever go with more than bolton parts, excluding SC and turbo setup, so i figured 3.91/4.10 would be very nice if one doesn't have SC or turbo. i wanna 4.10 so bad but i would hate to waste money i if get a SC down the road sometimes. so i am thinking 3.91 would be very acceptable.


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## jpalamar (Jul 22, 2008)

:seeya:


firebird said:


> see i am undecided if i will ever go with more than bolton parts, excluding SC and turbo setup, so i figured 3.91/4.10 would be very nice if one doesn't have SC or turbo. i wanna 4.10 so bad but i would hate to waste money i if get a SC down the road sometimes. so i am thinking 3.91 would be very acceptable.


If you went FI down the road I think anything over stock gearing would be overkill.


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## svede1212 (Nov 1, 2005)

firebird said:


> see i am undecided if i will ever go with more than bolton parts, excluding SC and turbo setup, so i figured 3.91/4.10 would be very nice if one doesn't have SC or turbo. i wanna 4.10 so bad but i would hate to waste money i if get a SC down the road sometimes. so i am thinking 3.91 would be very acceptable.


It doesn't take a lot to run near the limits of 3.91s. From a dig I'm in 1st gear about 2 seconds and in second a couple more.


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## GM4life (Mar 18, 2007)

firebird said:


> see i am undecided if i will ever go with more than bolton parts, excluding SC and turbo setup, so i figured 3.91/4.10 would be very nice if one doesn't have SC or turbo. i wanna 4.10 so bad but i would hate to waste money i if get a SC down the road sometimes. so i am thinking 3.91 would be very acceptable.


I think it depends on what type of FI you go with. I would think 3.91/4.10 would be over kill with a maggie. Being that my Maggied car with stock gearing makes 1st, 2nd worthless even with traction controll on. Gearing to .91/4.10's may not be so bad with turbos or centrifiguals with more mid range to top end grunt. Either way stubs and half shafts should be in order because of the torque multiplication.


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## firebird (Feb 6, 2011)

that makes sense, 1st is very itchy on a stock G. i almost squeezed the trigger on 4.10/3.91. thanks for openning my eyes guys. I'll thinking about it. how do you think 3.73would behave with tsv2300? i wonder now what gt500 has for the gearing in the rear, anyone knows? and how big of tires do they run on?


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## jpalamar (Jul 22, 2008)

firebird said:


> i wonder now what gt500 has for the gearing in the rear, anyone knows? and how big of tires do they run on?


3.55 with 285/40ZR18 rear tires.


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## GM4life (Mar 18, 2007)

firebird said:


> i wonder now what gt500 has for the gearing in the rear, anyone knows? and how big of tires do they run on?


GT500's came with 3.55 and 3.73's with 285 wide tires. They have traction issues too. The transmission was a TR-6060 with different gearing.


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## kellyx5 (Oct 20, 2011)

So what is the verdict? 391's for a normally aspirated G would be a noticeable difference that would make it a little quicker from the jump and not be terribly hard on parts or mileage, correct?


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## brianruben (Nov 23, 2011)

I want to do the 3:91s. any good shops in so cal?


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## firebird (Feb 6, 2011)

upping the gear is the best thing you can do if you are not planning on turbo, supercharger, or procharger, i know of some 04 guys with procharger with 3.91, not as bad as the 06 would be.

yeah i see gt500 had 3.55 and seems that ford will tone it down to 3.31 for the new line production, as well up the HP to 650.


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