Changed tire size how to correct speedo

I recently changed from the 75 series tires that came with to a slightly lower 70 profile, now my speedo is a bit off. Not a big deal yet, but I have a used axle with 323 gears that will be going in and throwing the speedo off even further. Does my 96 need different speedo gears, or do I need to have the computer updated?

Thanks,

Tom

That will take a computer flash. Very easy 10 minute job if you have the correct OBDII tuner for Lt1. I just did mine a little while ago when I went to 3.23 gears and 235-60-17 tires. If you do a highway milemarker test you can really dial them in to accurate for the odometer, but often the speed will still read a bit high from most speedos I have had, so you might have to chose which to get right on.

booster wrote:

That will take a computer flash.  Very easy 10 minute job if you have the correct OBDII tuner for Lt1.  I just did mine a little while ago when I went to 3.23 gears and 235-60-17 tires.  If you do a highway milemarker test you can really dial them in to accurate for the odometer, but often the speed will still read a bit high from most speedos I have had, so you might have to chose which to get right on.

I guess I need to get a tuner, or borrow one. Any recomendations?

Thanks,

Tom

There is a square wave modifier available for speedometers, but that will not help any ABS tone ring issues. You can set it for exactly the correct speed. The speedometer, odometer, cruise control, and computer will not know the difference.

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One thing that hasn't been mentioned that might be an issue is that the tuner, at least the Jet DST I have anyway, not only changes the speedo/odo outputs to correct them for the tires size, it also automatically changes the shiftpoints and lockup points for the transmission to match the gear ratio. In the program those transmission settings are in MPH, not rpm, so they move the setpoints to allow for that. Otherwise a change to deeper gears is going to move the shiftpoints up to a considerably higher rpm than they would be stock. Of course some people would like that and I think that is why sometimes it looks like the gear change gave even more acceleration and responsiveness than it really did.

Looking at the DST tuner on the Jet web site. Shows it is available fot the 96 caprice, but not available for the 96 roadmaster. I am fine with it updating shift and lockup points as long as they are reasonable, sounds like they can be overridden if they are wrong anyway. It' a big hunk of cash on my budget. Is the buick different enough to create a problem? I did have a lot of odb2 readers that didnt work with the buick, but worked on every other car i tried them on.

Isurf wrote:

Looking at the DST tuner on the Jet web site. Shows it is available fot the 96 caprice, but not available for the 96 roadmaster. I am fine with it updating shift and lockup points as long as they are reasonable, sounds like they can be overridden if they are wrong anyway. It' a big hunk of cash on my budget. Is the buick different enough to create a problem? I did have a lot of odb2 readers that didnt work with the buick, but worked on every other car i tried them on.

I actually managed to get an update to the DST that allows the Buicks to be done also. Jet bought the OBD version of TurnerCat and relabeled it as their own. Through a tuner guy I ran across, I got a contact at TunerCat who was able to write an update to the DST to put the Buicks in. He sent it to Jet and it is available to everyone that gets a DST, as far as I know. You might want to ask Jet about it, but should be no problem. It worked well, and the DST is better in one respect than the OBD TunerCat was in the the USB emulator is built in and that gets rid of the connecting issues the TunerCat had because of the difficulty in finding a compatible emulator.

So I can get the DST then upgrade it to work on the Buick? How much for the download? Do you have a link?

They did not charge at all for up update flash drive so you can install the update.  They sent me an email saying they had it and it showed up in the mail shortly after.

Spoke to a rep from jet today. He discouraged me from this direction. He said Once I started, I would have to program everything, and would probably make a mess. I like the idea of correcting shift points and lockup settings, not just correcting the speedo. I'm still thinking this is the right way to go.

Booster, were you able to change JUST the tire size and gear ratio?

Tom

Isurf wrote:

Spoke to a rep from jet today. He discouraged me from this direction. He said Once I started, I would have to program everything, and would probably make a mess. I like the idea of correcting shift points and lockup settings, not just correcting the speedo. I'm still thinking this is the right way to go.

Booster, were you able to change JUST the tire size and gear ratio?

Tom

You can change just the tire size and gear ratio, they are on the same page of the program so very easy, but there doesn't appear to be any way to keep it from adjusting the shiftpoints.   All and all that isn't as bad as it sounds because they basically appear to move about the same ratio as the gears and tires caused, which makes sense.  All it really does is give you somewhat the same rpm for the shift points as you had with the old gears.

If you do intend to change the shiftpoints and lockups that is one of the easier programming changes to do and it all is done in a few table.  You can setup upshift and downshift separately based on, IIRC, speed and TPS or maybe MAP.  Then you look at the lockup point and see if needs to move, usually to faster speed, so it doesn't overlap the upshifts.  I just kept moving them up until it felt responsive doing the kind of driving I do in daily driving first and then move up the rpm range after that.  I probably flashed it half a dozen times doing it all with each change taking about 20 minutes on laptop once I learned the program.

The guy from Jet is blowing smoke as the gear/tire change and then moving shiftpoints and lockups (if necessary, my lockups were not all that bad) is a very, very small part of all the settings in the program and some of the easiest to learn how to do. There is no need to touch anything else unless you want to modify other things like writing out the VATS, air pump, EGR or change the fan on temps and all those are pretty easy also. The hard stuff is in the engine control sections.

Changing the gears to a higher numerical ratio is going to lower the shiftpoints so what you may want to do is drive it a bit before you make the changes to the speedo settings for gear and tires to see how you like it with higher than stock shiftpoints.  If you like the way it drives you can just screenprint the shiftpoint and lockup tables before you make the speedo change and put the same data back in by hand after the auto change.

I have now spoken to reps several times. The best I can get from them is: If we did have an upgrade for the 96 roadmaster, we don't have it any more.

Would you by any chance still have it available to make, post, link, etc a copy?

Isurf wrote:

I have now spoken to reps several times. The best I can get from them is: If we did have an upgrade for the 96 roadmaster, we don't have it any more.

Would you by any chance still have it available to make, post, link, etc a copy?

I should have the flash drive here somewhere.  I could send you the files on the drive if you get a DST.  If they don't provide the update, which I think is ridiculous, the only other thing you can do is put a caprice PCM in it.  That is what I was going to do before I got the update.

On edit, it is also very possible that the update has been put into the new flash drives they supply with the DST. Customer service and tech reps are often way behind what is actually going on. I know they had/have the update as I got a Jet flash drive with it on it, from them. It would make sense they would just add it to file that they load when they make the install flashdrive.

If you can provide the upgrade, i will buy the DST
Thanks in advance

Isurf wrote:

If you can provide the upgrade, i will buy the DST
Thanks in advance

I found the Jet update flash drive.  It contains a single exe file for the update so should be able to send pretty easily by email. Date on it is 2020  and it is on a Jet labeled drive so this is the copy I got from them.

I would suggest that if you get the DST just hook it up per the instructions, which aren't the best, and see if it connects and reads the PCM.  If it doesn't connect it probably needs the update.  The instructions I had the wrong sequence of events to get it to read.  I had to open the first screen in the read/program software, turn on key, go to read or program pull downs, follow on screen directions.  My instructions had key on later in the sequence.

On edit: I just did a search in my old emails I keep around and I have the original email from JET saying they have the update and will send the update to me on a flashdrive. If I get can your email address, I will forward it to you if you want. Might be interesting for you to send it to JET to see what they say. I think it may be likely they updated their programmer file with update and it is included now in the install fall. They probably just never updated the literature to state that the Buicks are covered now.

Thank you!
Tom@wwworkz.com

OK
I have the Jet DST package. I tried plugging it in to the port last night with zero result. I have since applied the upgrade file provided. Before I go out to the car again, does it get plugged in and started with engine running of engine off? Do I start the software before or after starting the engine?

Thanks!

Tom

Isurf wrote:

OK
I have the Jet DST package. I tried plugging it in to the port last night with zero result. I have since applied the upgrade file provided. Before I go out to the car again, does it get plugged in and started with engine running of engine off? Do I start the software before or after starting the engine?

Thanks!

Tom

There are actually two programs to the software, the DST Tuner and Jet Flash. You will do the read and write things with Jet Flash. The tuner program is just for making the changes to the program in the a configuration file which is what the read will be.

Plug into the OBD port and into the laptop.

Open Jet Flash to the first screen only.

Turn on the key and shut off anything that is running, like the blower fan. Don't do anything until Passkey light goes out.

On Jet Flash go to tools/read. It should communicate at that point.

I had some of those same issues originally and it turned out to a that the program was not looking at the right com port so I had to change that. The com port selection is under setup in Jet Flash.