Speedo off

carneyman · July 7, 2006

So I washed my '02 z71 the other day and then parked it. My dad was the next one to drive it and he noticed the speedometer was off. Took it out and at 70, it says 74, at 75 it says 79, and at 30 it says 32. This just started and I have no clue what could be wrong with it. Any ideas/suggestions?

carneyman · July 7, 2006

Anyone?

HyperBuzzin · July 8, 2006

What were you comparing your speed to ?

Something that is recently calibrated and known to be exactly on ?

Have you checked tire pressure ?

4 MPH travelling @ 70 MPH (IMO) isn't that much off.

carneyman · July 8, 2006

What were you comparing your speed to ?
Something that is recently calibrated and known to be exactly on ?

Have you checked tire pressure ?

4 MPH travelling @ 70 MPH (IMO) isn't that much off.

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I checked it with a gps (and I haven't been pulled over yet), which is pretty accurate. It used to be exactly on.

Air pressure is 30, what I always run.

redvett · July 10, 2006

Fed. law allows MFR's +- 7mph for tires and other variations. You are very close especally considering tire wear and age of vehicle.

carneyman · July 10, 2006

Fed. law allows MFR's +- 7mph for tires and other variations. You are very close especally considering tire wear and age of vehicle.

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But I'm just wondering why it would all of the sudden change. I can understand a gradual change, but not in the span of a day. Same thing happened on my dads pickup, and now his will sometimes be on 60 when hes stopped. I would rather that not happen on mine.

redvett · July 10, 2006

Only one thing that can change that quickly and that is the cluster. Common problem. Find a GM authorized cluster & radio service center. They can fix the problem.

Draenor · July 10, 2006

Fed. law allows MFR's +- 7mph for tires and other variations. You are very close especally considering tire wear and age of vehicle.

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Where is this law to be found? I'd just like to give this to officer what's his nuts, if i get pulled over doing 1mph over.

(Yes it has happened and yes, I lost in court.) Sad isn't it.

carneyman · July 10, 2006

Fed. law allows MFR's +- 7mph for tires and other variations. You are very close especally considering tire wear and age of vehicle.

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Where is this law to be found? I'd just like to give this to officer what's his nuts, if i get pulled over doing 1mph over.

(Yes it has happened and yes, I lost in court.) Sad isn't it.

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I agree. I got pulled over for doin 72 in a 70...got a written warning. He walked up and asked me if I knew how fast I was going. I said "72", and he said "Yep. You need to slow down."

redvett · July 10, 2006

If you look in the FED. Dot. vehicle requirements you will find a requirement that all vehicles have to comply within +- 7mph. There is no way an honest police officer would ticket you for 2 mph when tire wear can offset your true mph.

That is called a speed trap.

You can fight it in court and win.

Many people have.

ZZebes · July 10, 2006

Hell i fought a ticket for 89 in a 55 in a work zone and won, i simply pointed out the the wonderful Texas DPS officer wrote down the incorrect vehicle and presto ticket be gone

carneyman · July 10, 2006

I tried to get out of a ticket where the cop wrote down the wrong intersection. Didn't work. Didn't help that the judge new me though.

toneman · July 13, 2006

If you look in the FED. Dot. vehicle requirements you will find a requirement that all vehicles have to comply within +- 7mph. There is no way an honest police officer would ticket you for 2 mph when tire wear can offset your true mph.
That is called a speed trap.

You can fight it in court and win.

Many people have.

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Methinks you should look up the legal definition of a speed trap while you're on the subject of looking at the federal DOT vehicle requirements. j/k!!!!

I do agree that 2 mph over is a bit extreme, but let's get real here--the tread on them tires have got to be pretty worn to result in a 7 mph difference between displayed and actual speed. In addition, if you were going to invoke that federal DOT speedo requirement, you'd in essence be acknowledging the possibility that your speedo might have been displaying an inaccurate speed--not a good thing to do if you're trying to argue that you're not guilty of speeding because your speedo was off; I mean, if you're gonna argue that you're not guilty because your speedo was off, you'd have to have been driving your vehicle w/ the speedometer showing a speed faster than the posted limit...in which case, why were you consciously attempting to travel at such a speed in the first place, regardless of whether or not the speed displayed in the speedometer is accurate?

With today's modern vehicles, correct-sized tires mounted on a vehicle w/ a properly-calibrated speedometer will not result in such a discrepancy in vehicle speed. Like I said, 2 mph is nitpicking but I guarantee you that more than a few cops will cite you for 5 mph over if they're using properly-calibrated radar...and you can argue that DOT rule all you want, but don't be surprised if you're still found guilty. After all, 5 mph over is 5 mph over...no matter what your speedometer may display otherwise.

BTW--I have 40K+ miles on my tires and according to my GPS, the speedo in my truck is just as accurate now as it was when those tires had less than 100 miles on them--okay, maybe it's a tenth or two off...but even then, it'd be hard to measure even that close unless your speedometer can accurately display tenths of a mile. That federal DOT rule may very well still be there, but I bet most of us would be hard-pressed to prove that our speedo was way off due to tire wear if using that argument to dispute a speeding ticket. Like I said, you'd have to have really excessive tire wear in order for the tire to affect speedometer accuracy by that much (7 mph) in today's vehicles...at which point I wouldn't be surprised if you get ticketed for excessive tire wear as well! Yes, tires wear differently (i.e., some tires will wear out in fewer miles)...but the result will still be the same--an excessively-worn tire, which in many states is considered a moving violation (just like a speeding ticket) rather than a mechanical violation...meaning, points on your record if you are cited for it and subsequently plead or are found guilty of such a violation.

redvett · July 13, 2006

Different tires of the same size from different MFR's are a different circumference and good honest Police Officers wont pull you over for less than 7 mph over. The stackup between the cruiser & and the speeder will be over that. The bad cops will do it and you will pay. Thats whats called a speed trap. You can beat the ticket but its going to cost you money. Best way is to not speed. Especially if you know of a trap. In Michigan if you go over posted speed by 10 mph your going to get it if you get caught.

toneman · July 13, 2006

redvett--I understand where you're coming from; my point (or argument, if you want to look at it that way) was that it'd be kinda hard for a driver to knowing acknowledge that the DOT 7-mph leeway exists, yet claim at the same time that he/she didn't realize that he/she was speeding if pulled over.

As far as whether LEOs would/should bother pulling people over for 7 mph or less--I'm sure they'll use their best judgment as to whether they should do so. However, keep in mind that more than a few cops might not be as forgiving if you're even just 5 mph in a school zone where children are present. Also, in some states (like here in CA) every once in a while (like on major holiday weekends) the Highway Patrol will run zero-tolerance enforcement on major infractions such as speeding...in this case, zero-tolerance as in even 1 mph over--and yes, they actually put out press releases stating exactly that prior to starting up their zero-tolerance enforcement. Now whether they'll actually pull people over for just 1 mph over is a different story, but I don't think they'd be saying that in the news just to blow smoke. After all, bad dishonest cops wouldn't be making such statements to the media, don't you think?

I'll offer one more thing regarding the DOT 7 mph leeway rule--if a vehicle's speedometer is going to read inaccurately due to tire wear, it should display faster than actual...meaning, the driver is knowingly travelling at such a speed (legal or otherwise) so that the speedometer is clearly showing a speed higher than actual. If you knowingly have the speedo pegged at a speed thats higher than the actual speed, then by default you acknowledge--if not admit--that your speedometer is inaccurate...in which case I hope you're able to cough up a good explanation to a judge as to why you would knowingly drive around w/ an inaccurate speedometer. And if you honestly didn't know/realize that your speedo might be off due to tire wear...then why would you have your speedo pegged at a speed higher than posted/legal? Again, tire wear will in theory cause the speedo to read higher than actual--it can never cause it to read slower than actual; the only way a tire can cause that is if the tire is larger than the tire size the speedo is geared for.

Bottom line--the reason the majority of cops don't bother pulling over most folks for speeding less than 10 mph over is that they're using their best judgment as far as whether or not said driver is a risk to himself and/or to others (I won't bother mentioning ticket quotas--that's for another discussion)...not because of some federal DOT standard regarding speedometer accuracy and tire wear. Like I said earlier, if some guy is going even just 5 over in a school zone where there are children--only a bad dishonest cop would not pull the guy over.