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      07-22-2019, 10:15 PM   #1
One28i
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Question thoughts on 3.73 fd ratio on 6spt MT 128i?

hi there,

I was wondering if anyone can share their thoughts on the swap to a 3.73 FD ratio from the auto 128i? any regrets? did you consider 3.64 or 3.91 options?

the car is occasion use as I work from home, but do drop the kids daily for a quick drive. Car does see much use, more street than highway but want it for light track duty as well

I hit the track a few weeks back, and the ratio for Cayuga Ontario road course were not too bad, was in 2nd in slow sections and would top 3rd in the fast sections with the stock 3.23 ratio. I could use some more power, as the acceleration in gear took a while to get to max speed, 3SI on the list as well.

thank you in advance for sharing.
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      07-23-2019, 08:30 AM   #2
Thunderguts
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I would do a 3.73 swap tomorrow if many of my miles weren't on the highway going 80 mph. It sounds like a perfect swap for you, however.

Our shifters aren't the fastest in the world. I can't speak from experience, but a 3.91 might have you shifting too often. I think the 3.73 would be perfect.
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      07-23-2019, 12:35 PM   #3
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I never looked back when I found out how easy it was to swap. It was my first mod, and the most impressive one you can do in my opinion.

Now if I had to do a lot of Interstate Driving...3:64 would be a tad bit better. But even still now when I am on roads with a 70mph limit...it's no issue.

But I also use to DD one of my built C5's with a 4:10 ratio.

Gears should the first mod for any car looking for a huge SOTP change. I then moved on to the 3SI, and then Headers....each one making the car that much more enjoyable. Milvs are next...and then I'm FBO.
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      07-27-2019, 11:11 AM   #4
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2009 BMW 128i  [5.00]
The 3.73 feels amazing blasting around town, but you have to shift a ton. Because of the need for an extra shift, the 3.23 almost certainly has a better 0-60 time, but numbers only tell a little of a story. You lose 2-3mpg on the highway, but I improved my around-town fuel economy from high 18mpg to high 19mpg. The shorter gearing is a blessing on the highway, it makes quick 5th/6th gear passes a breeze.

If you're going to swap the diff, I'd seriously look into buying an LSD. The shorter ratio, combined with the mechanical grip increase, changes the car's behavior like nothing else ever could (except maybe good suspension).
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