Calibrating Speedo needles

[Azazel]

Hey, I bought a reverse glow gauge face for my 99 SE and they came in as GT gauges. Difference being MPH. Now I have access to a “Snap On” Scan tool and I have the PCM Programmer ordered and on the way already. The question is will I be able to still use these gauges? I have them in right now and when I’m going 40 it says I’m going 70 :(. Just wondering if I have to send them back of if I can reprogram them with the Programmer or not? Thanks for all your help and I love this site.

[pyro]

send them back and get the SE ones, or just buy a gt speedo. to my knowledge no one has calibrated the different gauges.

[Sprucegagt]

You should be able to reprogram the PCM for the different gauge scale. What programmer are you using?

[harvester45]

Here's a question: my speedo is accurate at 55, but around 70 it is off by 4 MPH. Is there any way to adjust just the higher MPH settings?

To the first poster, I'm not sure that you can recalibrate the PCM, since I believe that the same signal is sent to the gauges, regardless of whether it is an SE or GT. If you changed any settings with the tuner, you're likely to throw off shift points or something else important. Can anybody confirm this?

Regardless, upgrading to the GT gauges or sending the gauge face back is your best bet.

[billytheman1188]

Why dont you just send them back and get the right gauges you need???? It would be much easier lol

[Sprucegagt]

Here's a question: my speedo is accurate at 55, but around 70 it is off by 4 MPH. Is there any way to adjust just the higher MPH settings?


Are your rims and tires stock size?

To the first poster, I'm not sure that you can recalibrate the PCM, since I believe that the same signal is sent to the gauges, regardless of whether it is an SE or GT. If you changed any settings with the tuner, you're likely to throw off shift points or something else important. Can anybody confirm this?


Completely and totally wrong!!!! You can recalibrate the speedometer in the PCM with HPT or DHP tuning software. It's very easy. Also it will NOT affect your shift points or anything else.

[Azazel]

I ordered the PCM Progammer off of PFYC.com so its the Digital Horsepower Inc. PowrTuner. I had it on order way before i ordered the gauges. And yes sending the gauges might be eaiser if i didnt live in Alaska. ;)

[harvester45]

Are your rims and tires stock size?

Yes. The discrepency is between the speedo and what my powertunr shows. So it is some kind of problem with the gauges (which are stock GT gauges). It is only 3 or 4 MPH at higher speeds, but still enough to bother me.


Completely and totally wrong!!!! You can recalibrate the speedometer in the PCM with HPT or DHP tuning software. It's very easy. Also it will NOT affect your shift points or anything else.

How? I know you can adjust for different rims/tires (pulses per mile or some setting like that), but I was not aware that you could recalibrate the speedo in a case like Azazel's.

[Azazel]

See thats what i thought. I know you can recalibrate for tire size and actualy needle fault. But as far as changing the speed that the needle moves because of the extra 30mph I wasnt sure of it.

[Sprucegagt]

Yes. The discrepency is between the speedo and what my powertunr shows. So it is some kind of problem with the gauges (which are stock GT gauges). It is only 3 or 4 MPH at higher speeds, but still enough to bother me.


You have something wrong with your gauges then. What the PT displays is what the PCM is actually computing.

How? I know you can adjust for different rims/tires (pulses per mile or some setting like that), but I was not aware that you could recalibrate the speedo in a case like Azazel's.

By adjusting the pulses per mile setting. It should be just a larger correction than for a tire/rim size change

[harvester45]

By adjusting the pulses per mile setting. It should be just a larger correction than for a tire/rim size change


Right, but adjusting the pulses per mile setting won't fix Azazel's problem, nor mine. He has the SE speedo, but he wanted to put the GT gauge face over the SE guts. What he wants is to be able to program the gauge cluster, not the actual PCM. There's a difference.

The PCM can be adjusted for larger/smaller outside diameters of the tires, but that is to correct the problem of the PCM reading the wrong speed. For Azazel to make his GT overlays read correctly, he would have to tell the car that he has smaller wheels. The PCM will send the appropriate signal to the car's instrument cluster, and the needle will move less, as you have rightly ascertained. There is a catch, though:

The speedometer needle's sweep is the same for both the 120 MPH SE and the 150 MPH GT gauge clusters (180 degrees). So when his car is going 120, it would show up on his new overlays as 150 MPH. To correct this, he would have to increase the pulses per mile by 20%. That can be done, and it would make his speedometer read accurately, but the PCM would "see" him going 20% slower than his actual speed (remember that the signal is sent in a standard voltage per MPH to the instrument cluster, which is what makes the SE to GT gauge cluster swap possible). Do you see what I'm trying to say?

Aren't shift points based in part on MPH? So if the PCM is "seeing" 20% slower speeds, wouldn't it hold out on it's shifts longer? Maybe I'm not being descriptive enough, is any of this making sense to anyone other than me? lol

[Sprucegagt]

I see what your getting at now and your right, it would cause problems. Thanks for the catch! :thumbs:

[harvester45]

No problem!

[Azazel]

So there is no good way to do it? Just send them back and get the right ones sent this time. *sigh* alright, just another shipping bill. Thanks for all the help though.
Also where could i find the GT cluster if i wanted to scrap sending the faces back and just go with the whole GT thing.

[Azazel]

Just so everyone knows. Some buddys and I are going to attempt to use the PCM to program it and adjust for the changes as an experiment. Ill let everyone know how it goes.

[harvester45]

I'd check out e-bay for a GT cluster. If it was the company's mistake, I would return the overlays, and ask for a free return label. There are a few GTs being parted out that you could try too. I know there is at least one on this site, and another on GAOC. I'd advise trying that first. Otherwise you're looking at a lot of needless programing headaches.