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Old 07-29-2004, 04:06   #1 (permalink)
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frown Diff ratios and comp (auto gear changes)

if say i change the gear ratios on my car (ef futura) by getting 3.45 LSD and change the speed sensing thing, so it shows the right speed, will the gearbox change at the right rpm or will it carry on further till it reaches the instigated "km/h"

so if i plant it, will it change when it gets to 5500rpm or whatever, or will it just go up to the limiter? :(

as i heard the comps takes readins from speed (km/h) rather then rpm in order to make the changes, if so, is there a way to correct it?
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Old 07-29-2004, 05:19   #2 (permalink)
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Re: Diff ratios and comp (auto gear changes)

Quote:
Originally Posted by lx^el
if say i change the gear ratios on my car (ef futura) by getting 3.45 LSD and change the speed sensing thing, so it shows the right speed, will the gearbox change at the right rpm or will it carry on further till it reaches the instigated "km/h"

so if i plant it, will it change when it gets to 5500rpm or whatever, or will it just go up to the limiter? :(

as i heard the comps takes readins from speed (km/h) rather then rpm in order to make the changes, if so, is there a way to correct it?
It more likely wouldnt make much difference because your not really moving that far from what you have now (3.23)
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Old 07-29-2004, 05:24   #3 (permalink)
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Re: Diff ratios and comp (auto gear changes)

i've got a feeling i've got a 3.08 not 3.23 single spinner (as it cruises at like upto1800rpm@100km/h)
but not 100%, will have to check tommorow morning
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Old 07-29-2004, 06:32   #4 (permalink)
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Re: Diff ratios and comp (auto gear changes)

look at it this way, the computer only sees tailshaft rpm, it has now idea how fast you are moving down the road, so if left with std drive gear, it will change at the same rpm regardless of diff ratio... you could put a 10:1 diff in and it will still change gears at the same rpm, even though you might only get 15kmh out of first gear
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Old 07-29-2004, 14:44   #5 (permalink)
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Re: Diff ratios and comp (auto gear changes)

lx^el

You will have a 3.08 diff and a brown 20 tooth speedo drive gear by rights.

To keep the speedo reading correct change to a grey 22 tooth speedo drive gear part no. XE17271E.

This will mean your speedo reads within 2% correct and your speed limiter will still be at 180kmh.

The Auto will change 1-2 and 2-3 at the same old speeds but now that will be at higher revs to get to those speeds with the 3.45 diff. Fortunately this will be a good thing with a modded engine that likes to rev and the 10% higher revs to change up is still just under the redline 5600rpm and well short of the rev limiter 5900rpm. I am not sure about the diff housing and accessory mountings between the diffes.
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Old 07-29-2004, 17:16   #6 (permalink)
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Re: Diff ratios and comp (auto gear changes)

1800rpm at 100kmh sounds like 3.23 to me, thats what my car does since my recent 4 speed auto and 3.23 diff conversion
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Old 07-29-2004, 18:54   #7 (permalink)
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Re: Diff ratios and comp (auto gear changes)

Quote:
Originally Posted by ebs_4l
look at it this way, the computer only sees tailshaft rpm, it has now idea how fast you are moving down the road, so if left with std drive gear, it will change at the same rpm regardless of diff ratio... you could put a 10:1 diff in and it will still change gears at the same rpm, even though you might only get 15kmh out of first gear
Anyone who has had the transducer fail on a V8 will tell you the car does not change gears very well at all (I know - my speedo went dead and the car wouldn't upshift). The computers are set with gear ratios in them (that's why I purchased an ELGT ECU from NZ - 3.45 ratio).
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Old 07-29-2004, 20:35   #8 (permalink)
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Re: Diff ratios and comp (auto gear changes)

An EF with 205/65R15 will do 1810rpm at 100Kmh. With a 3.23 its 1900rpm at 100Kmh.

Thats real actual Kmh not speedo reading which won't change when you change diffs.

You dont need to change ecu just the speedo drive gear and it's only $8.

If you don't change the spedo drive gear the car will change up as normal but you will hit the speed limiter at 158Kmh ( of couse the speedo will read 180) and at 100 on the speedo your only going 88kmh.

So your 0-100 will improve out of site!!!
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Old 07-30-2004, 23:03   #9 (permalink)
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Re: Diff ratios and comp (auto gear changes)

thanks everyone for replies...

i guess i am off to look for 3.45 lsd now :D
(had a real cheap one lined up, but turned out the prick at the wreckers doesn't even know what he has or hasn't got in stock... and i had to drive half way accross town to be disappointed)
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Old 07-30-2004, 23:03   #10 (permalink)
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Re: Diff ratios and comp (auto gear changes)

Quote:
Originally Posted by jonbays
An EF with 205/65R15 will do 1810rpm at 100Kmh. With a 3.23 its 1900rpm at 100Kmh.
where did you actually get these figures from?
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