Speedometer Problem

**jcws6** · 09-27-2009, 07:58 AM

I recently replaced the rear end gears in my Trans Am (went from 4.10 to stock 3.42). Nothing else was changed, and the tires are stock size. I took my car to a shop to get the speedometer calibrated, and they ran into a problem.

The speed reported in the PCM matched up with the speed on the dyno. But the readout on the speedometer in the dash was off. The shop owner's son, the dyno guy, and I were all confused about why the hell that was happening.

Thoughts?

**Sweet Chops** · 09-27-2009, 10:41 AM

I wonder if you could just pop the needle off and adjust it to where you you need it. Or you can program a taller or shorter tire size into the PCM to get the readout correct.

Ryan

**jcws6** · 09-27-2009, 02:10 PM

Originally Posted by Sweet Chops

I wonder if you could just pop the needle off and adjust it to where you you need it.

Can anyone confirm the needle thing?

The previous owner put some white gauge faces on the instrument cluster, so I asked the tech if maybe the guy screwed it up. He said that he was pretty sure the speedo needle only went on one way & that it was impossible for it to be adjusted like that. He wasn't 100% sure though, so if you guys know, I'm all ears.

They ended up adjusting the tire size in the PCM so that the speedometer is exactly right at 70, but 3mph off at 35. I didn't ask them to screw with it too much, because I'm planning to put 4.10s in the car next year. But I'd like to figure out what's up with the speedometer, in the meantime.

**GONZO SS** · 09-27-2009, 03:06 PM

if they popped the needles when they did the gauges its most likely the problem. that's y a lot of companies r making the overlays were u don't have to pull the needles

**jcws6** · 10-11-2009, 08:54 AM

Originally Posted by GONZO SS

if they popped the needles when they did the gauges its most likely the problem. that's y a lot of companies r making the overlays were u don't have to pull the needles

Damn. Any recommendations on how to fix it?

Also, if the needles were off, wouldn't the speedometer be off by the smae amount at all speeds? Right now, it seems like the speedometer is off my a percentage (i.e. the faster I go, the less accurate it is).

**35th-ANV-SS** · 10-11-2009, 09:07 AM

Originally Posted by jcws6

I recently replaced the rear end gears in my Trans Am (went from 4.10 to stock 3.42). Nothing else was changed, and the tires are stock size. .................................

.............................I didn't ask them to screw with it too much, because I'm planning to put 4.10s in the car next year. But I'd like to figure out what's up with the speedometer, in the meantime.

Why did you go from 4.10 to 3.42 just to go back to 4.10 again next year? I'm a little confused......

And, I agree with above. The needles were pulled off when doing the white faced gages and weren't put back on in the correct position.

**Sweet Chops** · 10-11-2009, 11:17 AM

Why didn't you report that you had white face gages in your first post? How else are you going to get the gages on without taking the needles off?
I'm going to take a guess that the car had 4:10s when you bought it. I bet the guy put the needle wrong to compensate for the 4:10s.

Ryan

**jcws6** · 10-11-2009, 01:43 PM

Originally Posted by 35th-ANV-SS

Why did you go from 4.10 to 3.42 just to go back to 4.10 again next year? I'm a little confused

I bought the car with 4.10's installed.

They broke. So I put a used stock rear in it, which was actually cheaper than new gears (go figure).

Those will probably break, so I plan on getting a 9" someday.

With 4.10's. That hopefully will not break.

**jcws6** · 10-11-2009, 01:46 PM

Originally Posted by Sweet Chops

Why didn't you report that you had white face gages in your first post?

Because I reported that I had white gauge faces in my second post.

The dude at the shop said it was an easy install & very hard to screw up. I doubt the previous owner was smart enough to mess with the needle on purpose, since when the tech looked at my PCM, it was not calibrated for stock gears.

**35th-ANV-SS** · 10-11-2009, 01:49 PM

Originally Posted by jcws6

I bought the car with 4.10's installed.

They broke. So I put a used stock rear in it, which was actually cheaper than new gears (go figure).

Those will probably break, so I plan on getting a 9" someday.

With 4.10's. That hopefully will not break.

Ah, well that makes sense now. +1 on the 9". I have the Moser 9" with 4.11 gears.

**Sweet Chops** · 10-11-2009, 03:25 PM

So is it fixed now, Or you going to just pop the needle off and put it in the correct place.

Ryan

**jcws6** · 10-12-2009, 11:22 PM

Originally Posted by Sweet Chops

So is it fixed now

Nope. The dyno techs could never figure it out. The best thing they could do is screw with the tire size, but the speedometer is still off at low speeds.

I'm not sure exactly where I need to put the needle. It seems like it's already resting as far down as it can go when the engine's on & the car is sitting still.

Plus, I'm STILL not sure that the needle would cause the speedometer to be a percentage off (i.e. the speedometer is right at 65mph but ~3-4mph off at 25mph).