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Gear Change and PCM BCM

Mattwood440

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For Pete's sake can someone explain how changing the rear gears can be accomplished without drivability issues? I'd like to start a thread that addresses year to year changes, auto vs manual, other option interference, max and minimum ratios done, and what the heck needs to be done to which control modules and why. It shouldn't be this hard. I've now been told three completely different things since I had mine done over 2 years ago. Mine went perfectly and needed no tune but I think I am in the minority.
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WildHorse

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15-17 Manual trans cars with gears between 3.15-3.73 & tires up to 31" nothing needs to be done.
15+ auto trans cars with gears between 3.15-3.73 & none stock tire heights needs a tune.
18+ manual cars with gears between 3.15-3.73 & none stock tire heights + rev match needs a tune.
At least, that's my understanding now.


Edit: nevermind see below answer.
 
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K4fxd

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15 + auto's are limited to 3.57 rear ratio.
15 + manuals are limited to 3.73

If you get a FRPP tune it allows a 4.09 ratio.

With a tune you can put any gear you want as long as your tuner changes the correct boxes
 

Sweetface

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prior to getting my gears done I contacted PBD since i had their e85 tune. They stated I needed a tune update to have 4.09s installed . Today i had the gears installed and with updated tune uploaded everything went perfect . big shout out to the guys at PBD and Delk Performance. Love the E85 / 4.09 combo .
 
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Mattwood440

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15-17 Manual trans cars with gears between 3.15-3.73 & tires up to 31" nothing needs to be done.
15+ auto trans cars with gears between 3.15-3.73 & none stock tire heights needs a tune.
18+ manual cars with gears between 3.15-3.73 & none stock tire heights + rev match needs a tune.
At least, that's my understanding now.


Edit: nevermind see below answer.
I wouldn't cross this out. We need specificity.
What about 15+ auto trans cars with gears between 3.15 - 3.73 & stock tire height?

This is good info that needs to be shared, because a sales rep for a very popular tuner shop who I won't mention said in 2018 I needed to buy their proprietary tuner for approx $1000 to go from 3.73 to 3:55 in my 15 manual. I didn't.

15 + auto's are limited to 3.57 rear ratio.
15 + manuals are limited to 3.73

If you get a FRPP tune it allows a 4.09 ratio.

With a tune you can put any gear you want as long as your tuner changes the correct boxes
And is this a tune to the PCM only? I ask because a couple years ago everybody was telling me BCM. Besides the FRPP one, is this a custom tune only, or can this be done with an off the self Diablosport or SCT?
 

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K4fxd

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From the factory the PCM gets a signal from the BCM telling it what the gear ratio is. The highest (numerically) accepted ratio is what I posted. AFAIK there is no lower limit, although it would not surprise me if you put 2.73's in it might throw a code.

Tuners have 2 options for setting gear ratio. They can disable the auto sensing and manually input the ratio or they can raise the ratio limit.

As long as the ratio is within the limits, IE 3.73 or less on a stock manual, nothing needs to be done. The car will auto correct and your speed o meter will read correctly. Also you will not get any codes.

If you put 3.91's in that same car you will need a tune.
 

WildHorse

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As long as the ratio is within the limits, IE 3.73 or less on a stock manual, nothing needs to be done.
Good.
I went from a 3.31 / 27.3" tall tire to 3.73 / 28" tall tire.
 

K4fxd

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That should be fine
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