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Speedometer - TIRE & WHEEL SIZE

dna9656

Full Member
Joined
Sep 5, 2016
Messages
250
Location
Puget Sound, USA
OK; when I bought my '06 it has 22" wheels and low profile tires on it. Uggg. I thought I understood why the speedometer was off, it was 9 mph off. I put 245/70/17 tires and 17/7.50" wheels on it. The speedo is STILL 9 mph off! So what's up with that?
 
It's about the outside diameter of the tire, wheel size has nothing to do with it.
 
That size tire is not even 31" tall. Sounds like the truck was running some BIG tires at some point & the speedo was adjusted. Any descent shop or dealer should be able to make the correction for you.
 
MS03 2500 said:
It's about the outside diameter of the tire, wheel size has nothing to do with it.

The near stock tire size is way smaller than those 22" low profile jobs. Apparently the speedometer is not the conventional spinning cable and magnet  (I knew this) so it must be electronically adjusted. The indication is not dependent on the transmission revolutions at all but from some other source to a computer.
 
Still is a speed sensor mounted to the transmission as I understand but instead of driving a cable it drives a sensor which feeds to the computer and the computer sends a signal to the cluster and a motor in the cluster moves the needle... Ahh progress...

Rodney
 
Most handheld tuners should let you adjust your speedo.  The Supertuner I had would do it, that is until someone broke into my truck and stole it.  :mad:
 
There is a really good site with a calculator that will compare 2 different tire sizes and indicate the speedo difference between the 2 at any given speed (use Tire Comparison tab). Try:
https://tiresize.com/calculator/

Using your 245/70/17 as one size, the previous person must have had the speedo adjusted for a 34 inch tire (315/70/17 for example). That will yield a 9 mph difference at 70 mph. But...

Remember, the mph error is relative to any given speed, the slower you're going the less difference, faster equals more difference.

So, if you are 9 mph off at ALL speeds, your gauge cluster may have a bad stepper motor. When my was going bad, the speedo would only bottom out at random numbers, say 15 mph, so it would indicate 15 mph high at that time.

BTW, stock size for that year with 17s was 265/70/17.
 
I went from a stock 245/75/16 to a 265/70/17 so I'm off 2 mph a 60. To be off 9 percent is really off ???
 
How are you measuring it. Do you have a a scanner
 
Oh I never have tried that, so I'm not sure how accurate it is.
 
Most phone GPS coordinates are +/- 25 ft, at least the Iphone 6 anyway, that leaves a lot of room for error.
Outside of that, as Dalefan mentioned, it's physically impossible for it to always be 9mph off. Time/Distance is not a linear relationship to an increased or decreased OD. of a rotational object, the more revs/minute (the faster you travel), the larger the degree of error. You can't possibly be 9 MPH off at 30MPH and also 9MPH off at 65 mph. The laws of physics make this impossible.
Even if your cluster/speed sensors we reset for a different tire size, the relationship to that tire size would still be a variable percentage based upon the revs/mile of the programmed tire OD.
I would think that this is what you are experiencing, a reset/reprogrammed reading based on an alternate size tire to the one you are currently running, not simple a linear MPH error.
 
We use cell phones to calibrate ski boat speedos all the time. The margin of error is not enough to really matter.
Like the others asked, what speed are you seeing the 9 mph error?
 
The speedo starts moving WAY below the beginning of the scale. I'm doing what feels like 10ish when it reads 0 to 3 mph. On my way to work is a radar sign telling you what speed you're doing, there are several others around town. When I go by one of these signs I show 25-26 and the radar shows 35 to 36. In one 15 MPH limit there is a radar sign, I do an indicated 12 it screams 22 to 23 MPH. On the freeway I show an indicated 50 and keep up with those in the slow lane, I do an indicated 60 and I'm cruising with the "got to be first" crowd and they usually do 70 to 75 some times 80. I have confirmed this on my motorcycle and it has an old fashioned magnet and cable set up; it's off about 5 mph according to the same radar signs and co-workers that get behind me and "suffer" by doing the limit...they say even though the sign says 35 the limit is really 40, well if that were true the signs wouldn't read 35. So it's pretty weird to me how the error seems to be pretty constant, I agree it flies in the face of the logical way it should be working but it is what it is. I pay close attention to my speed as I drive though heavy woods filled with un hunted deer, they will dart out in the road, the cars behind me bunch up so close to me... if a deer does enter the road I hope i have time to drive off the road because if I hit the brakes I will get run over by the cars behind me so I drive NO faster than I have to in hopes that we all will be going slow enough that the cars behind me can stop in time.
Call me paranoid but I'm alive.
 
dna9656 said:
The speedo starts moving WAY below the beginning of the scale. I'm doing what feels like 10ish when it reads 0 to 3 mph.

Sound like the gauge is bad.

As I mentioned earlier, if you are off the same at ALL speeds (which you are), chances are that your speedometer stepper motor is going bad. Gen 1 AVs are known to have this issue. There are multiple threads on this board about this problem. A new cluster will set you back between $100 and $200. You can ship it off for repair, ~$50, or you could tackle it yourself (google it, lots of videos and instructions out there).

Removing the gauge cluster is super easy, putting in a replacement takes less than 30 minutes.
 
Dalefan said:
Sound like the gauge is bad.

As I mentioned earlier, if you are off the same at ALL speeds (which you are), chances are that your speedometer stepper motor is going bad. Gen 1 AVs are known to have this issue. There are multiple threads on this board about this problem. A new cluster will set you back between $100 and $200. You can ship it off for repair, ~$50, or you could tackle it yourself (google it, lots of videos and instructions out there).

Removing the gauge cluster is super easy, putting in a replacement takes less than 30 minutes.

I disagree.  In my experience when the stepper motors go bad the gauge readings aren't consistently off as in this case.

I believe someone has replaced a stepper motor or motors on this cluster already.  To do that all of the gauge needles need to be pulled off of the stepper motor shafts and then pressed back on during reassembly.  It is not straightforward to get the needles properly oriented when putting them back on - my water temp gauge is currently reading high for this very reason but to me it isn't worth the trouble of pulling the cluster and resetting that needle - I know how much it is off and roll with it.  If it was the speedometer I would take the trouble of getting it right.

I suggest removing the cluster, note where the speedometer needle is pointing, lift the needle with a fork, and then pressing it back on the shaft such that it is pointing to a position counter clockwise from where it is now at an angle representing a difference of 9 mph.

Do a such on this web site or on YouTube for stepper motor replacement on GM truck clusters.  Obviously you can skip the part where they are unsoldering a stepper motor and soldering in a replacement but just about everything else will apply, other than you only need to lift the speedometer needle not all of the needles.
 
I just have to ask as this is could be the possible issue. In the DIC personal Settings, are you indicating KMH insisted of the desired MPH?  ???

The school zones comment throws me this way.

Where I live 85 miles froth Mexican Border, we have many vehicles, trucks and cars from down there just poking along and I finally figured out they were using KMH and always oblivious to the problems they were causing and constant horn honking.

You can usually see this on your Speedo Dial in the subset of the analog face.

Most cars are set to be a few miles slower than indicated so the Manufacturers are not selling vehicles showing the Speed to be lower on the dial than you are actually traveling so they are not at fault if you receive a citation and were just then taking the new vehicle home from the lot. I've always wondered if we presented a vehicle to the dealer for warranty work with 35,975 miles on the clock and be told they checked the computer and discovered we were actually closer to 37,00 miles.  :eek:

When I used to take my vehicles to the AAA Speedo checks, I'd be showing 1 or 2 MPH under at 25-35 MPH and at 60+, usually 3 to 5 less, and it dropped off even more at 70-75 MPH.

Driving in Utah back in the day of the 85 MPH limit, and with GPS, I was showing that I was barely doing 78 MPH. I never figured out if this was from Digital Lag and the early days of personal GPS units.

Your Milage May Vary :D

With my 2010 Avalanche, I was at 35,925 miles and had to go to the dealer to get a part for another vehicle. I knew there were some TSB's to resolve and when I spoke with my MR GoodWrench Guy and explained I had 3 days of busy stuff to do and expected to be putting 75 to 100 miles onto truck. He told, "Drive Backwards".

I went ahead and let hm do the work right then. While this was going on, I perused the lot and knew they had 2 4x4 2013 Black Diamonds in stock. When the 2010 was finished, I was told by my very well known to me Mr GoodWrench Guy when he pulled me aside and said this dealer wanted my truck a whole lot more than I wanted a 2013.

It tuned out, he was not wasn't kidding.  :eek:
 
  I ordered all 6 stepper motors from Dr. Speedometer, he included 4 light bulbs with the

order for $38.and change through Amazon.com.

A friend of mine who is a former electronics tech for the USN soldered them on for me.

I followed the www.youtube.com videos to the letter.

Now my problems are worse.

The volt gauge keeps turning clockwise until the motor reaches the end of it's travel, the

oil pressure gauge remains largely the same, sweeps clockwise it it hits 80PSI and then

goes to zero, my wife tells me the speedo is now erratic and the temp maxes out too.

So now is all worse.

Any ideas?
 
Ouch. I'd be questioning the work your buddy did. When I sent my gauge cluster in, the speedo, fuel gauge & oil pressure gauge were all acting erratic. One day they'd be fine, then out of the blue one of them would be bouncing all around, then the speed would be way off, then the gas gauge would say I'm almost out, then theyd be fine for no apparent reason. When the cluster came back, everything worked great. Almost 2 yrs later, they still work fine.
 
If you were closer to me I'd like to see via my HPTuners scanner what the computer is seeing regarding voltage and temp.  Those two may be a needle placement issue.  Oil pressure may be a bad sending unit, the speedometer sounds like you may have gotten a bad stepper motor, but once again it would be nice to see what the computer thinks the speed is to isolate the problem to the cluster or its power/ground connections.  Maybe someone near you has something that will allow you to read some of these parameters off of the OBDII port???
 
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