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Old 08-07-2013, 06:06 PM   #1
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Default rear gear question 1995 trans am 3:23->3:42 speedo change in PCM?

i'm pretty sure know the answer on this as i think i saw it done before, but i'm having the rear end rebuilt on my car and the shop said it was easier to get 3:42 gears instead of 3:23 and they cost the same so i decided to do the upgrade. i know the speedo/odo will be slightly off and the shop doesn't have the equipment to get into my computer but i think another shop down the road does that doesn't work on rear ends. i have friends who know the owners of both shops so they said after we get the car done at shop #1 it can be driven down to shop #2 for the fix. i told them i'm pretty sure it's just a PCM adjustment but wanted to ask you guys to make 100% sure.
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Old 08-07-2013, 06:09 PM   #2
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Yeah, you can drive as long as you want, it will only throw the speedometer off. I haven't had mine tuned yet because I'm waiting on new cam to go in..
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Old 08-07-2013, 06:11 PM   #3
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can't imagine it takes more than 10-15 mins to get changed. i doubt a shop would charge a full high priced tune just to change the speedo gear. i saw someone do it in a 95 camaro PCM using a 94 camaro (95 camaro pcm swapped to 94 so they could use the old obd1 cable) in about 10 minutes.
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Old 08-07-2013, 06:15 PM   #4
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I wouldn't think they would.
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Old 08-07-2013, 06:20 PM   #5
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do you think i made the right choice by doing the 3:42 upgrade? my car is basically a cruiser but it will be nice for just a little bit more acceleration. i didn't want to go crazy like 3:73 or 4:10 but i figured 3:42 would be worth it even if it ends up costing me $40-$50 more than 3:23 (for the speedo calibration) any more than that i'd probably just spend the $60 for the cable and do it myself. i think there are free LT1 edit programs you can use for minor tuning like that?
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Old 08-07-2013, 07:04 PM   #6
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You didnt do anything wrong. Like you said they cost the same and the shop could get them easier. The 3.73's would've gave more thrill but if you just cruising nothing wrong with 3.42's.
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Old 08-07-2013, 07:52 PM   #7
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yeah i didn't want 3:73 cause i hear that hurts mpg pretty bad on the highway.


oh and i have a question... did the factory rear ends come with blue markings on the diff cover and marks inside? the shop says they think my rear has been worked on before but i have owned the car for 9+ years and other than changing the rear diff seal and the pinion seal several years ago i don't think the rear has been opened since i've owned it. the previous owner bought the car new and had all the service done at the dealer so i doubt there would have been a used rear end installed at any point.
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Old 08-08-2013, 05:49 AM   #8
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not sure about that one..
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Old 08-08-2013, 07:25 AM   #9
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you'll increase the torque to the rear axles by 6%.

see
Torque change at axles due to rearend gear ratio change

miles per hour change will be approx 1mph per 10pmh.you'll be showing 63.5mph where before you were showing 60mph
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Old 08-08-2013, 11:25 AM   #10
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well that should be getting reprogrammed. unfortunately it turns out my carrier was bad and now a $700 repair is a $1200 repair ugh.
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Old 08-08-2013, 05:49 PM   #11
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http://reddevilriver.com/aldl.html - You can get either the 12 or 16 pin version. $55 shipped to US locations - This is me

C.A.T.S. WinFlash - Up and download the .bin file from/to the PCM - 30 days free then $19.95

TunerPro RT - Used for scanning and for programming the .BIN file - Free and runs 100% after a 10 second 'nag' screen. Purchase the license and you can avoid the nag screen.
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Old 08-08-2013, 05:56 PM   #12
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IMO its a waste of time to go from 3.23's to 3.42's. With an overdrive transmission the 3.73's would have been just fine for you. Not sure what the shop has to do to get into your pcm to recalibrate your speedo. Get a power programmer and you can do it yourself along with playing around with other adjustments like shift firmness and rev limiter.
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Old 08-08-2013, 07:09 PM   #13
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i have a hypertech but it's got someone elses VIN in it. i'm not interested in spending $300 to get the speedo calibrated

Quote:
Originally Posted by phonedawgz View Post
http://reddevilriver.com/aldl.html - You can get either the 12 or 16 pin version. $55 shipped to US locations - This is me

C.A.T.S. WinFlash - Up and download the .bin file from/to the PCM - 30 days free then $19.95

TunerPro RT - Used for scanning and for programming the .BIN file - Free and runs 100% after a 10 second 'nag' screen. Purchase the license and you can avoid the nag screen.

i assume i'm the 16 pin type as i have a 95 with the OBD2 connector under the dash. i actually thought about trying to adapt the serial connector on the other end of the hypertech cable i have into usb but i don't think such a connector exists.
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Old 08-09-2013, 01:15 AM   #14
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$1200 yikes you're takin it up the butt fella.
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Old 08-09-2013, 12:08 PM   #15
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Well they ended up having to replace the whole posi unit. ended up being $1308 with axle bearings. The one shop cant read the pcm because they dont have the tools and the dealer wants $100 so i think im going to order the 16 pin cable for $55 and use the free software to program the pcm myself.
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Old 08-09-2013, 12:26 PM   #16
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Ok ordered the 16 pin cable. Going to try it myself.
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Old 08-09-2013, 01:00 PM   #17
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Also you will accrue more miles on the car than you really are until you get your speedo recalibrated.
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Old 08-09-2013, 01:48 PM   #18
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Yeah i ordered the cable and already have a laptop so hopefully with those 2 free programs i will be good to go. Its not my dd so i doubt i will put too many miles on it before i recalibrate it.

btw i snapped a pic today of the car up on the lift. this is the rear diff paint the shop says means the rear was used. it had an auburn unit in it which they said wasn't factory. i thought the factory units were auburns?

Click the image to open in full size.
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Old 08-09-2013, 07:29 PM   #19
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Agreed. Waste of money. When I did mine back in the day 3.23 to 3.73 I didnt notice much. 3.73 10 bolt to my new 4.30 8.8 build "oh my god". Also t56.
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Old 08-09-2013, 07:50 PM   #20
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i didn't spend any money to go from 3:23 to 3:42. well not really, i guess $55 for the tune, but the rear end needed rebuilt and 3:42 were easier to locate than 3:23 and didn't cost any more.
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Old 08-09-2013, 07:50 PM
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