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Old 08-26-2009, 09:27 AM   #1
m60e30
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who can help with gear/diff ratios relating to rpms!

ok guys, need to get some imput here, something just isnt adding up.

ok, got me a 5 speed gear box in my e30 ratios are
1- 4.20
2- 2.49
3- 1.67
4- 1.24
5- 1.00

those are pretty low gears, but im using a 2.93 diff, (or i thought i was anyway), but this thing revs like nobodies business.
so i guess my question is, does anyone know how to calculate what my revs should be using this combo?? cuz i thought i would have revs to spare.. the only thing i could see being the problem is its not a 2.93 diff?

the trans is out of a e34 530i, and apparently they used a 3.07 diff in the manual cars...so my taller diff should not be rev city. i can just throw it in second gear for all take offs and its like the perfect first gear.

anyone? anyone?
oh my wheels are 22.5 inch diameter
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Old 08-26-2009, 09:50 AM   #2
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I use this site:

http://www.unixnerd.demon.co.uk/revs.html
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Old 08-26-2009, 09:55 AM   #3
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Hah, funny you post this now, i'm actually looking at the exact thing atm to see if the getrag260 will do well with the 4:10 diff.

http://www.diffsonline.com/techinfo/...Calculator.xls
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Old 08-26-2009, 10:46 AM   #4
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I've been using this one:

http://ctny.audiworld.com/mark/s4/gears/gear_calc.html

I calc that in fifth you should be turning 2760 RPM at 100kph with a 2.93. With a 3.73 it would be 3500 RPM (ouch).


(BTW, those are quite undersized tires. E30 should be around 24". )
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Old 08-26-2009, 10:58 AM   #5
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ya thats what i calculated as well. hmm, well i better check my tach first see if its acurate, cuz those are the numbers the tach is telling me, about 3400ish at 100km. and it feels that high to. ill hvae to double check this diff. i took a 3.73 out of it!
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Old 08-26-2009, 11:01 AM   #6
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Sounds like a 3.73 to me. Maybe you put the wrong one back in.

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Old 08-26-2009, 11:22 AM   #7
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and i dont know it my tach can be off that muhc. i mean the e34 530 with that exact same trans used a 3.08 diff, and on a 5 series i dont know if bmw would hvae made a first gear that rediculously low, maybe its for hill climbing?
hmm, ok thanks for the help. ill check diff and rpms tonight.
sure is lively though with this gearing! haha
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Old 08-26-2009, 12:41 PM   #8
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If you swapped from a 6-cyl to a V8 and didn't do anything to the tach then it's entirely possible that it's way off. Pretty sure you need to get a coding plug for a 4-cyl and rig up some sort of resistor setup to make it read right.
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Old 08-27-2009, 07:35 AM   #9
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calculator

http://wahiduddin.net/calc/calc_speed_rpm.htm

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