# What is your experience with rear gear swaps?



## gtogooch (Dec 12, 2009)

I am looking to maybe change the rear gears in my 04 M6 Goat. What have you guys gone with? Is it worth it? Hows the mpg? What ratio did you go with? My car is a daily driver and I take the interstate to work, but I would like it to romp off the line still! Thanks in advance.


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## 87GN06GTO07IRL (Aug 10, 2009)

I went with 3.91's in my m6 and couldn't be happier. Lost 2mpg tops, but it pulls so much harder it's worth it to me. Once i go f/i, i will put the stock gears back in. If your over 450 lb/ft torque at the wheels i would stick to stock gears or you'll just roast the tires.


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## gtogooch (Dec 12, 2009)

Good info, I was thinking about the gears but was worried about it being too much gear for road use. Its hard to get fat tires under the backs of these cars anyways so I want to find the medium between melting the tires and getting good traction.


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## svede1212 (Nov 1, 2005)

The hard part is getting the right wheels so they are wide enough for the tires and have the right offset to center them in the wheel well. With 9" wide wheels and a 50mm (+or-5mm) offset you can get 275-285s in the back with full suspension travel.


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## 04gtslow (Mar 22, 2012)

is that with out rollong the rear finder


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## svede1212 (Nov 1, 2005)

Mild roll


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## danfigg (Sep 27, 2009)

*re*

Back spacing of the rim is also part of the equation. I see that my 17X9 inch camaro rims have a backspacing of 6.875 inches. That measurement is taken from the mounting flange of the rim. Lay the rim flat and measure from the flat mounting surface to the edge of the rim. Offset is the measurement of the mounting surface in relation to the rims center (standing up). Zero, positive, and negative. In other words. If the mounting surface is inside (such as deep dish rims) than it is a negative offset. If the mounting surface is towards the outside of the rim (No lip) than that is positive offset. and if its in the center, than that is a zero offset. So with that being said 48 mm to 50 mm of offset and backspacing of 174.625mm is just about enough for the rim to fit with minor rubbing on the outer quarter. If I machine the mounting flange and remove 4.455mm from the mounting flange, that will give me 179.07mm or 7.05 inches of Backspace moving the rim inboard to give more clearence to the outer quarter. Once we have the correct measurement, then we will know exactly how much offset and backspace is needed for a 9 inch wide rim and 275 tire. We should be able to do this with no need for rolling clearencing the rear quarters..... Thats if my math is right. ----Danfigg


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## svede1212 (Nov 1, 2005)

Because of 1" widening on stock wheels and adding 5mm spacers to bring them back out a little I ended up being 55.7mm effective back spacing (48mm stock + 12.7mm - 5mm). An inch wider adds 1/2" (12.7mm) to offset and an inch (25.4mm) to back spacing. Your roughly 5mm adjustment should be about right. Even with pulling the tire in (by removing bulge) and increasing offset (and therefor back spacing) it was still a little close to the fender for me to go without a mild roll. The difference of sidewall height also plays into it as on corners a higher sidewall has more room to move sideways (17>18>19). Tire brand will make a difference tho too. With the same brand a 275 to 285 is 5mm (roughly 1/4") wider on the inside and outside.


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## danfigg (Sep 27, 2009)

*re*

Oops sorry highjacked your thread this is about gears not rim and tires. I would say since you want best of both worlds I would suggest 373's. Know back to tires, Gear ratio and tire height make up your final ratio. The stock size tire is about 25.7 inches high. To much gear and you will have to compensate with a taller tire. As you know we dont have much room for that. I did have a 27 inch tire on my car with no issues. it was a 235/55/17. I think with the 346' ratio and the 27 inch tall tire, it made my car sluggish off the line. A lot of people say 373' are not much different from our stock ratio. In a drag race situation, I would say with 373 you would be crossing the finish line in the high 4000 RPM range maybe 5000 RPM. At cruise speeds maybe 1800 RPMs at 60. Just estimating-------Danfigg


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