GSus

iTrader: (2)

My speedo is off...way off. Had a "GM certified" mechanic look at it, and re-calibrated it. Said it's now dead-on, however, it's exactly 10mph off....i.e, 50 on speedo is actually 60 etc...
I don't know how he could plug it in, and then be so adement(sp) about the accuracy. He said the original discrepancy was only off by 2-3 mph.
So, giving him the benefit of the doubt, and understanding that he's under the impression that the speedo is now reading accurately; what would the odds be of the problem being the vehicle speed sensor? No trouble lights are on. Also, if it is the vss, where's it located? I've read that it's either on the crank...or tranny tailshaft. I have the t56 btw.


I apologize for the crummy description (and the brutalizing of the english language), typing this through my phone.

T.I.A folks

Michael02hawk

iTrader: (26)

Unless a dyno, radar or laser speed measuring device is used in conjunction with someone reading the speedometer while driving, its not possible to detect a speedo being off.

The dealer probably just re-flashed the PCM with the standard tune.

Your speedo is most likely off due to a gear or tire size change.

In which case you'll need a custom tune to handle those changes.

white2001formula

iTrader: (5)

When the dealer replaced my rear end due to an accident, they put in a 3:23 ratio rear end. When I was showing 70 on the speedometer, I was actually doing 58. My grandmother actually asked me why I was driving so slow. (True story)

I'd say that you have a gear change like said above. I'm not sure going to a bigger tire would throw you off that much either, such as going to 18-19-20.

GSus

iTrader: (2)

I don't think a gear change happened, of course I haven't jacked the rear up and started counting revolutions either.
Tire sizes are (3) 275/40zr17, (1) 255/40zr17...it came like that, I know, I need to change it...cont.

GSus

iTrader: (2)

I didn't think the size difference would matter much (not at least by 10mph) considering the smaller tire is on the front. steering kinda sucks...but not DD'ing it right now.
Also, dunno if this correalates, but my tach isn't accurate either... a little on the low side, if that makes sense.

Thanks for the responses...keep em coming

GSus

iTrader: (2)

ttt for further insight.

white2001formula

iTrader: (5)

275 or 255 represents section width. The 40 represents a sidewall aspect ratio which is a percentage of the the section width. This section width will be how tall your sidewall is physically.

Example: (made up numbers)

275, 9" width 40=3.6" Sidewall height
255, 8" width 40=3.2" Sidewall height

Your tire height is shorter on the 255, and the width is smaller. (width will not affect speedometer rating though) This would probably affect your steering.

I agree, 10mph is quite a bit off. I'd venture to guess that the PCM needs to be programmed to a different gear ratio. I changed mine to 3:23, because it was a known option for Auto's, then had a friend drive 70mph next to me and problem solved.

If the gear size had changed and tuned prior in the past, and a GM Dealer reflashed it back to the VIN, it will flash it with what the vehicle was geared with stock. I don't know where the VSS is, but I think there are only a few things that can throw off the Speedometer.

-Gears
-Tires
-VSS
-Different geared transmission (going out on a limb here)

GSus

iTrader: (2)

Definately things to consider.

Thank you very much for the explanation

GSus

iTrader: (2)

I forgot...the other day, my buddy and I measured the height of the 2 front tires.

the 255 was 1/2" shorter than the 275. Different manufacturers may play a role in this. I should've mentioned that a couple posts back.

wssix99

iTrader: (5)

...yea. You need to get a new tire. The 275/40zr17 are correct but the other one gives you an unsafe set. It should be .7" shorter in diameter and the width is also smaller by .8"

Is the smaller tire on one of your rear wheels? If so, the differential should compensate side to side, but the speed sensor will behave like it sees two rear wheels that are .35" shorter than they should be.

GSus

iTrader: (2)

The smaller is on the front. I'm not going to drive it until I get it replaced. Hell, I still need to spin the rear by hand to see what gears I have...weather's been shitty here lately.

wssix99

iTrader: (5)

Then the circumference of your rear is at stock and you've got something messed up on the inside.

I like to check my spedo with a GPS unit. Usually if you put it in map mode (not navigating) and drive down a straight road, it will give you an accurate speed. (No need for radar guns, etc.)

I'd also suggest you look at getting a scan tool that can read your PCM's readings. If your PCM gives an accurate speed and your speedo doesn't, then you may just have to reposition your speedo needle. Some people have had this issue when taking their gague needles off for an aftermarket inlay. What does yours look like?

eseibel67

iTrader: (10)

It is in fact very easy to check actual speed.

Borrow a handheld GPS. They are very accurate.

white2001formula

iTrader: (5)

+1 to this. I didn't even think about this situation.

GSus

iTrader: (2)

Well, it just so happens that I was looking at the needle the other day. It looks fine EXCEPT, at rest when car is shut off, needle fully rests as far down as it can. The '96 I use to have, the needle stayed perfectly at zero. I would love to ASSume that there was never an overlay installed, as the previous owner (original) was a 55 year old mortgage broker. Maybe someone can post a pic of their cluster with vehicle shut off for comparison. I think the tach sags slightly also...might explain its inaccuracy. Might be a good thing if there was an overlay on...then it'd be just a matter of repositioning the needles.

I don't have a gps...or direct access to one either.
I DO have however, a tuner buddy who could probably take a look at it if it starts leaning towards a pcm issue...let him start tickling that keyboard, I might walk away with some goodies. However, he does live a little ways away.

Great responses everyone...thank you all

wssix99

iTrader: (5)

That's normal. The needle rests on the plastic - so you can't really tell if its off or not at rest. Having someone check the PCM speed vs. looking at what the needle says should be telling.

GSus

iTrader: (2)

That's what I get for being optimistic...that just took the wind out of my sails.
Whatever happened to the good ol' fashioned cable and gear setup. Sometimes it seems as though technology takes a step backwards...sheesh.

Ok, so I would probably be better off just having my buddy plug in, and drive around to compare? If the pcm's accurate, then the problem would lie soley within the cluster...right? And if that were the case, then my only other option would be a new cluster? Damn, I wish 6litereater would chime in also...his site says he has a hook-up who actually recalibrates the cluster themselves.

Thanks again

wssix99

iTrader: (5)

Yea - but using a GPS would also help a lot, if you can find a friend who has a portable automotive unit.

You probably won't need a new cluster. The spedo is a very basic electronic component and would rarely get screwed up. Your needle might just be out of place. In that case, you just pop it off and then back on in the right position - but you need to localize the problem first.

If PCM = Spedo AND PCM/Spedo <> GPS THEN you have a PCM issue or a gearing issue. (We've ruled out wheel/tire issues.)
IF PCM <> Spedo AND PCM = GPS THEN you just have to move the needle
IF PCM <> Spedo AND PCM <> GPS THEN things are really screwed up