Jeremy95LT1Form

Just had my 2.73 rear changed to 3.73. The guy who installed it is using tunercat to fix the speedo and give it a mild tune. He changed the settings so the speedo was correct but stated it was shifting all "messed up." So he set the rear back to the 2.73 tune and changed some other settings for fan temp and leaned out the fuel to air a little. now it runs fine, shifts ok, but the speedo is off about 20 to 25mph. He said he doesn't know what is wrong, thinks it might be a problem with the computer, which i don't think it is as everything else is perfect, and he needs to do some research. If anyone has some advice I would appreciate it. I can't stand having to use my GPS to get my correct speed and constantly seeing close to 100mph on my speedo when cruising with traffic.

Thanks in advance.
Jeremy

bobdec

Jeremy, he kept the shifting characteristics the same by not correcting the speedometer settings. Here is some reading " https://www.camaroz28.com/forums/sho...d.php?t=498673 " Once you change the rear drastically, 2.73 to 3.73 is a 37% RPM to speed difference. AND you correctly set the speedometer settings then you need to alter a few tables in the A4 tune to get the 4l60e shifting at the right time. EG: with 2.73 the 2-> 3 WOT shift is set for 91 MPH, about 5200 RPM. At 91 with 3.73's rear you would be about 7K RPM. So you have to lower 2->3 shift MPH down 37% into the low 60's to keep this thing below the 5800 cutoff. Basically you need to lower your Manual, Normal, performance, cruse and kickdown up/Down shift points 37% across the board. Then look at you TCC enable/disable tables and see how you want your TCC to function.

shoebox

All he should have done is correct the speedo. If he did that correctly, it should shift fine. Stock shifting is based on speed and if the speedo is correct, the shifts will be. No need to mess with anything else, unless you just want to. The guy probably does not know what he is doing and now he really has messed it up.

WS6T3RROR

If you use the wizard for this in tuner cat to correct the speedo it will adjust the shift tables to keep the engine rpm that the shift occurs at the same. Unfortunately this can make the car seem to short shift really bad. Which will require some tweaking to make you happy with how it drives.

Jeremy95LT1Form

I will have to talk to him and see what he has figured out. I don't know what he did, but I'am gonna have him put everything back to stock, then be there with him when he makes the changes.
Thanks again for the help.
Jeremy

MikeGyver

He needs to multiply the MPH shift points by .73 (2.73/3.73 = .73)

Regearing changes the rpm to mph corelation.
Resetting the tire size/gear ratio will correct the speedometer but the shift points will be off as the previous mph will no longer match with the rpm. Shift tables are RPM vs MPH.

WS6T3RROR

If he uses the wizard in tunercat it will do the scaling of the shift tables if you hit apply and then hit apply again at the second prompt.

shoebox

Partial throttle tables are TPS vs. MPH. Kickdown tables have the RPM limits as I'm seeing it.

bobdec

WS6, I was not aware of the 'wizard' so I tried testing it by hitting apply a couple of times under the speedo tools routine and nothing. Is that a Tuner OBD2 only feature ? or do I have a bad version. I'm running Tuner 2.1 (OBD1) with definition file $EE (PCM_EE) version CM. I have always had to do the shift tables manually as changing the speeod settings does not alter any of my normal,performance,manual, cruise shift tables. It would be good if it did, always willing to learn something new.. Bob D..

NJ-LE

bobdec:
"I'm running Tuner 2.1 (OBD1) with definition file $EE (PCM_EE) version CM"

They're up to 2.23 on the OBD1 tuner and $EE def is "CS". The update is said to be free and does appear to be fully functional in rev 2.23.

WS6T3RROR

Time for show and tell I guess. This one is going to be going from 3.23 gears to 3.73 gears step by step.

First go here.


Then this pops up and you put in the info that is correct now and hit calculate and then apply.


Then this will pop up right after you hit apply and you should click apply again.


And save the new file denoting the gear change. Open it and compare it to the original file in all the tables. They are all now lower to keep the rpm the shifts occur at the same even the kickdown tables and lockup. I also put my version information on there.

MikeGyver

You're right. I wasn't very clear... MPH and RPM must be met to command a shift, at WOT anyway, part throttle I guess it's just MPH.

So the MPH data in the shift tables must be rescaled (the same amount as the gearing change) as the previous RPM will no longer generate the same MPH.

MichiganSkip

X2 I went from 2.73's to 4.10's and just changed the speed calibration. Shifts just fine.

bobdec

OK.. I upgraded to Tuner 2.23 and now get the frames that match WS6 posted..The older 2.1 did not have the pop-up option to alter the shift tables after the first apply hot button. Thanks..

shoebox

That's something that Tunercat owners should stay up to date with...updates.