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Need tune after diff ratio change

Skoti69

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Hey guys,
Ive recently changed the diff ratio in my vx commodore (ls1) from the standard 3.08 to 3.73, so now my speedos out by 18 percent or so and the shift points aren't right. Just wondering if anyone here could suggest a good place to get the speedo sensor ratio retuned to suit the new gears?
Im located in the south east suburbs of Melboure.
Cheers in advance :)
 
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EYY

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Shift points don’t change - the vss is in the trans so it will just think that the speed is higher than it actually is. Shift points are determined by perceived road speed and are set according to useable rev range. It’ll still kick down the same when it needs to - it won’t need to be retuned for just a diff gear change.

Only real issue is the speed that the torque converter will lock up at. It’ll lockup sooner with 3.7’s because the pcm thinks road speed is higher.

Having said that, I had 3.9’s in for a very long time and never noticed a difference apart from it locking up slightly sooner. The additional load at lower rpm is offset by the greater mechanical advantage given by the shorter gears anyway.

Imo, just change the ppk settings in the cluster and leave it at that. 3.7-4.11 gears are crap for highway driving, only because you don’t have the legs to kick back into second to overtake like you do in 3.08-3.46 gears. The car will drive very differently with 3.7’s and will take a while to get used to. For example, mine won’t kick down into 2nd at 90kmph with 3.9’s, but can manually select second and will hit the limiter at 6500 at about 125kmph. I’ll be going 3.46 next time.
 

_R_J_K_

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If you're confident in tackling some really basic wiring, you can get speedo corrector boxes that tap into the VSS signal wire and are pretty inexpensive compared to what a shop will bill you. They have a couple of dials you use to change the speedo until it's correct (just use GPS on your phone as reference).
 

Skoti69

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Shift points don’t change - the vss is in the trans so it will just think that the speed is higher than it actually is. Shift points are determined by perceived road speed and are set according to useable rev range. It’ll still kick down the same when it needs to - it won’t need to be retuned for just a diff gear change.

Only real issue is the speed that the torque converter will lock up at. It’ll lockup sooner with 3.7’s because the pcm thinks road speed is higher.

Having said that, I had 3.9’s in for a very long time and never noticed a difference apart from it locking up slightly sooner. The additional load at lower rpm is offset by the greater mechanical advantage given by the shorter gears anyway.

Imo, just change the ppk settings in the cluster and leave it at that. 3.7-4.11 gears are crap for highway driving, only because you don’t have the legs to kick back into second to overtake like you do in 3.08-3.46 gears. The car will drive very differently with 3.7’s and will take a while to get used to. For example, mine won’t kick down into 2nd at 90kmph with 3.9’s, but can manually select second and will hit the limiter at 6500 at about 125kmph. I’ll be going 3.46 next time.

Cheers for the info! What im noticing is really sluggish gear changes when first taking off from being completely stopped..then as soon as it hits 2nd it flies. So just that inital take off is really all im worried about atm !

Ill definitely look into what you've said.. Was also in the process or researching a new speedo gear for the trans but struggling to determine how much teeth i have currently and then need to change to as there is so many options!
 
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Skoti69

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If you're confident in tackling some really basic wiring, you can get speedo corrector boxes that tap into the VSS signal wire and are pretty inexpensive compared to what a shop will bill you. They have a couple of dials you use to change the speedo until it's correct (just use GPS on your phone as reference).

Thanks mate, actually in the process of doing that right now haha.

Just a bit confused in regards to where to wire it as theres so much conflicting information! Ive got it connected to the blue white wire from the speed sensor before the pcm, but from now what i think ive gathered it instead needs to he the purple white one after the pcm? Thanks for the reply :)
 

EYY

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Cheers for the info! What im noticing is really sluggish gear changes when first taking off from being completely stopped..then as soon as it hits 2nd it flies. So just that inital take off is really all im worried about atm !

Ill definitely look into what you've said.. Was also in the process or researching a new speedo gear for the trans but struggling to determine how much teeth i have currently and then need to change to as there is so many options!
It has a reluctor wheel - you don't change speedo gears, its all electronic. Just find somebody with a scantool so they can change the PPK value. Shouldn't cost you any more than $50 to have it changed.

VSS won't have an impact on the performance in first gear.

What else has been done to the car? Have you had it tuned?
 

Lex

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It has a reluctor wheel - you don't change speedo gears, its all electronic. Just find somebody with a scantool so they can change the PPK value. Shouldn't cost you any more than $50
When l changed my diff ratio to (3.46) the guy (mechanic) l use, charged me $30.00 to change the ppk (pulses per kilometer).
 

Skoti69

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It has a reluctor wheel - you don't change speedo gears, its all electronic. Just find somebody with a scantool so they can change the PPK value. Shouldn't cost you any more than $50 to have it changed.

VSS won't have an impact on the performance in first gear.

What else has been done to the car? Have you had it tuned?

No worries, all starting to make a bit more sense i think, i had read on a few other threads that you can't just change the ppk, but good to have reassurance that a cheapish option will do the trick.

Ive done a fair bit of maintainace work eg just swapped burnt out 34 clutch pack fot a new one but not to much performance wise. (eg tuning)
Might also be worth mentioning its duel fuel :)
 

Skoti69

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When l changed my diff ratio to (3.46) the guy (mechanic) l use, charged me $30.00 to change the ppk (pulses per kilometer).

Awesome ill start having a look around for some local tuners, thanks for the info! =)
 
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