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Old 01-11-2021, 11:30 PM   #1
Rat Dangerous
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Default Rear end ratios with m46 knowledge required.

So I have a 1994 north american 940 N/A with an automatic and a b230fd. Reading online it says my rear end had a gearing ratio of 4.10. I am buying a 1990 740 with a m46 and b234 which also has a 4.10 rear end that I will be swapping (the m46) into my car and making an automatic b234. I am wondering if a 4.10 rear with a b230fd and m46 will have too crazy of a gearing for actual comfort of driving as this is my first time doing something like this. Please let me know your driving, shifting, and rpms with swapped m46s and rear axle ratios. With my calculations I can cruise with a 4.10 at 105 km/h (normal canadian highway speeds) at 3000 rpm
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Old 01-12-2021, 12:23 AM   #2
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If they both have the same rear end drive the 740 with a light throttle and see how you like it
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Old 01-12-2021, 04:31 AM   #3
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First gear would be pretty good for towing a house off it's foundation, other then that should work.
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Old 01-12-2021, 07:34 AM   #4
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I run 4.10s and early M46 in my 245. It's definitely short. I like it, but I'm not too concerned with comfort. Sounds like you might have already done it, but there are calculators you can find with a google search that you can plug in the ratios of the trans and diff, tire size, and engine speed and get your mph at that rpm in each gear.
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Old 01-12-2021, 11:56 AM   #5
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I have a 3.73 rear end with my B230FT and M90 and wish for longer gearing every single time I drive my car. Stock 3.31 rear end with the M47 in the 240 feels like an appropriate match. You can always drop down a gear, after all.
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Old 01-12-2021, 09:44 PM   #6
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My V8 Wagon has 3.31 gears a 2 speed Powerglide and lots of HP, used to run 3.90 gears, the motor was all ways wound out on the highway . What are the final drive ratios in the gears,[example, My MGB with O/D, 3rd gear direct,=5.13/3rd gear O/D= 4.11,4th direct= 3.90 4th O/D= 3.20. Unless you have HP 3.31 works for me with the V8, tend to gear it with a 3.54/3.70/3.9? As a side note , coming back from Snowmass, Co. years ago coming over the divide 30+ miles UP, 4 direct could not pull the hill[3.90] 3rd O/D could [4.11]
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Old 01-12-2021, 11:23 PM   #7
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354 is the magic number.
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Old 01-13-2021, 12:12 AM   #8
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354 is the magic number.
I tried to sell a low mileage complete rear end and no one wanted it for $50, to the great crusher it went.
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Old 01-13-2021, 12:57 AM   #9
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3.54 is a good one for sure. I bought one that had a g80 installed in it with new bearings and such at the IPD swap meet a few years back. Complete 740 rear end with 3.54 gears and g80 for 200 dollars. Not sure who Culberro isn't selling rears to, but he never asked me. Ha. Put in it my 90 factory 745 manual turbo wagon with 550k miles on it. The original 3.54 rear the gears where worn out real noisy. I had put a 3.31 in it but went back to the 3.54 with my bmw getrag swap which has same gears as m46 pretty much. I'll probably put a 3.31 in my SE when I put the M90 in it just because I have it and 3.54's are hard to find.
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Old 01-13-2021, 11:29 AM   #10
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Quote:
Originally Posted by FROSTED_BVTTS View Post
I run 4.10s and early M46 in my 245. It's definitely short. I like it, but I'm not too concerned with comfort. Sounds like you might have already done it, but there are calculators you can find with a google search that you can plug in the ratios of the trans and diff, tire size, and engine speed and get your mph at that rpm in each gear.
Early m46 has a better 1st.

I had 3.73 and m47. 1st was ridiculous.
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Old 01-13-2021, 12:03 PM   #11
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3.73 for best combo of fun, cruising and economy
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Old 01-13-2021, 03:57 PM   #12
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Quote:
Originally Posted by redblockpowered View Post
I have a 3.73 rear end with my B230FT and M90 and wish for longer gearing every single time I drive my car. Stock 3.31 rear end with the M47 in the 240 feels like an appropriate match. You can always drop down a gear, after all.
Just buy taller tires next time, con would be your speedometer would be off but you'd also put on less miles.
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Old 01-13-2021, 05:24 PM   #13
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3.73 is too too low geared for me and is slower acceleration than the 3.54
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Old 01-13-2021, 10:57 PM   #14
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I like 3:73 with an M46 and a turbo, ya 1st is useless but it is no matter what you do.

4:10 is great if you like shifting a lot and losing top end.
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Old 01-14-2021, 01:33 AM   #15
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I have a 3.31 and an M47 and its fine. I personally wouldn't go any shorter than a 3.54. I probably wouldn't enjoy a 4.10
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Old 01-14-2021, 01:34 AM   #16
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I was relatively happy with 3.73 rear gearing and my M47 (similar to a later M46). Although 1st was rather short and as a result the car felt a bit slower off the line because I had to shift so soon. I am inclined to agree with others in this thread. 3.54 seems like a nice compromise. One of the reasons I was looking at a T5 for that car was because of the taller 1st gear.
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Old 01-15-2021, 06:47 AM   #17
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Tire diameter changes the math.
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