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Old 12-24-2004, 03:01   #1 (permalink)
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3.9:1 diff ratios.. On road usage?

Chasing any peeps that are running a diff with about mentioned ratios.

How do they go around the streets, besides real hard. what are the revs like for normal driving.

Fuel consumption i spose is the big question, is the extra launch worth the fuel, due to engine revs?

Does it sit on 110kph with a decent rev amount.. i dont want it to be screeming all the way to the city,, if you know what i mean.

Will be driven by a warm 250 c/f with 200 rods. (high comp). 5 speed, and N2O for weekend draggin.. in an XF Ute
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Old 12-24-2004, 06:02   #2 (permalink)
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Re: 3.9:1 diff ratios.. On road usage?

Dont have 3.9's. i got 4.11's not to bad for city, but highway your pretty much screwed. mine sits on 2400 @100km approx. fuel economy does deffinately drop but then i went from a 2.92 to 4.11's. try it an if you like it make your own decision on if its worth the extra fuel.
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Old 12-24-2004, 21:51   #3 (permalink)
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Re: 3.9:1 diff ratios.. On road usage?

with 4.11's on the strip do u need to change into 5th?
i always though with 3.9's and a 5spd in a 4.0ltr u are hitting redline in 4th across the line?
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Old 12-24-2004, 21:54   #4 (permalink)
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Re: 3.9:1 diff ratios.. On road usage?

Does any one have a calculation to figure out top speed in each gear with each different diff ratio? I want to higher as well but not sure if I want to use my overdrive to get across the line. Im doing 168km/hr now across the line that is with 3.27's not sure how many revs I am at though.
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Old 12-25-2004, 00:18   #5 (permalink)
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Re: 3.9:1 diff ratios.. On road usage?

there was an excel spreadsheet going around, i think walki might know? it had all the standard ratios for the t5 (incl different 1st ratios) and even the 4spd i think.
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Old 12-25-2004, 19:11   #6 (permalink)
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Re: 3.9:1 diff ratios.. On road usage?

Yeah Walkinshaw has that spreadsheet I'm sure..

2400rpm @ 100km/h isn't that bad really. The auto XF's would sit on around 2100-2200rpm @ 100km/h so you're only talking a few hundred revs more. My EA sits on 2500rpm @ 100km/h and I get better than 400kms to a tank so its not terrible. I often sit for several hours on end at 2700rpm and fuel consumption really isn't that bad.

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Old 12-25-2004, 19:50   #7 (permalink)
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Re: 3.9:1 diff ratios.. On road usage?

i have 3.7 , 3.9 , 4.3 and other ratios .all for 9" all motive gears
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Old 12-25-2004, 23:46   #8 (permalink)
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Re: 3.9:1 diff ratios.. On road usage?

I haven't had the chance to do strip yet (I keep snapping to many axels), dont ask me how i know but 4th now maxes out around the 140-150 mark as my speedo is off i tell exactly. So im guessing i would b well in2 5th by the time i get across the line.

Fuel econ has drop since my car used to rest at around 1800rpm at 100 when it had a 2.92.
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Old 12-25-2004, 23:54   #9 (permalink)
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Re: 3.9:1 diff ratios.. On road usage?

Something to be aware of is that with a 3.9 diff, the tailshaft rotational velocity increases greatly at the same speeds, so whereas they are not recommended above 180km/h, that comes down to something like 140-150km/h.

Just something to consider in the name of safety.

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Old 12-26-2004, 01:46   #10 (permalink)
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Re: 3.9:1 diff ratios.. On road usage?

good point axeman, but most people should have a tailshaft loop by now, or i hope they do
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