When the manfacturer engineered the vehicle they designed it for the 245/75-16 tires Changing the tires to a 265/75-16 tire changes its diameter approx 1,5 inches and is wider too. Therefore the circumference is bigger and changes your speedo. There are ratio adapters that screw onto the speedo gear but i'm not sure about the ones in the transfer case. The chip method does work. If you intend to keep that size tire then maybe a gear ratio change is in order. I would note the tire size change on the door data plate too. Here is a calculator for trans gear ratios and gear color sizes.
https://www.tciauto.com/speedometer-gear-calculator

 

He put on a tire with a larger circumference which would make his speedo read SLOWER. Taller tire has lower RPM (revolutions per mile), yet his speedo problem is it's reading faster than he's going. That means with the stock tire size, it would read even faster yet. I'm having a hard time believing with that tire size the speedo is off 5mph FAST. I know everyone thinks GPS it the be-all and end-all, but GPS available to non-military users is not as accurate as GPS can be. I can tell you that making the same tire switch on mine which amounted to a 9% reduction in RPM (Tire Rack shows RPM for specific tires and not all the same size are truly the same size), and effectively changed my axle ratio from 4:10 to 3:73, made my speedo spot on going by those radar speed signs that are set up here and there to show you your speed. I would try to find one of those, or take a stopwatch (your phone has one) on a highway with mile markers.

 

You have to reprogram the ECM for the tire size your running now. The gear in the trans thing is old school. Not applical in your truck. As for the chip, IDK about that ether. I do know my 97 silverado speedo was off, and I used a jet programmer to reset it. You can get a programmer just for speedo adjustments for about IDK maybe $150. My programmer is a performance flash tuner. It around $300. Thing is, if you buy a programmer, you can always reprogram for different tires.

 

Your new tires will give you about 4% slower tire speed. (corrected, I had a brain f#@*t.
If you want a wider tire, the correct tire size is 265/70 - 16.

 

Your new tires will give you about 4% faster tire speed and not the other way around.
If you want a wider tire, the correct tire size is 265/70 - 16.

A taller tire will read SLOWER on the speedo because it turns fewer revolutions per mile.

https://www.discounttire.com/learn/...accuracy#targetText=Up-sizing, or installing a,than the original equipment tire.

 

In the old days before GPS we would gauge a speedo by driving down the HWY @ 60 MPH, and time the mile markers. 60 seconds between mile markers @ 60 MPH is spot on. As for it being off to start with, well all the gauges work off of stepper motors. IDK id they get weak when they get old in some way, but I was running OE size tires in my 97, and my speedo was off. Never changed rear end gears, everything OE. I recentily swap my cluster to a 120 MPH Danili cluster, and it was off much more. I was able to correct it by adjusting tire size with my programmer. I know others have done the same cluster swap, and did not need to reset there ECM to read the rite speed. I also have need to adjust my speedo for tire size when I put my last tires on. They are 275/60/15. My OE size is 235/15/78

 

View attachment 134464 View attachment 134466 In the old days before GPS we would gauge a speedo by driving down the HWY @ 60 MPH, and time the mile markers. 60 seconds between mile markers @ 60 MPH is spot on. As for it being off to start with, well all the gauges work off of stepper motors. IDK id they get weak when they get old in some way, but I was running OE size tires in my 97, and my speedo was off. Never changed rear end gears, everything OE. I recentily swap my cluster to a 120 MPH Danili cluster, and it was off much more. I was able to correct it by adjusting tire size with my programmer. I know others have done the same cluster swap, and did not need to reset there ECM to read the rite speed. I also have need to adjust my speedo for tire size when I put my last tires on. They are 275/60/15. My OE size is 235/15/78
View attachment 134458 View attachment 134462

Hi Ken,
I've found some things on the web about the stepper motors in the instrument cluster.
I believe I have an Air Core F type stepper motor for the Speedo.
They tout the new steppers better because they use silicon lubricant , they state that the lube in the originals hardens up.
I'm going to just leave it alone for now, have to get my daily driver into a shop for A/C repair.
My daily drive is 40 miles each way, the grocery-getter gets 29 MPG on 87 octane.
The Suburban is running Premium and gets about 17 MPG.

Have a great day !

 

My 99 owned since new has always showed faster on the speedo than GPS. Depending on the tires used, it has been 3-5%.

As the tires age and wear down, the variance increases - no rocket science there.

The same, to varying degrees, for all other cars owned in the family fleet over the years and compared with a GPS (several Volvo's, several Saab's, a Jag, a BMW & a Saturn). They too show variation depending on the tire mounted at the time and state of wear. FWIW, snow tires often show the biggest discrepancy and the Jag was the most accurate of the lot.

While you can fool around to chase accuracy, IMO, the simplest method if you want to show an accurate speed is to buy a clone ELM 327 OBD scan tool with bluetooth or wifi and then create gauge set using the (free) torque app. Under $30 to buy, 10 minutes to set up and done (you really don't need the ELM 327, as the phone gives you the GPS, but it is way cool to get real time data on other things). I just use an old phone in a Ram mount,

 

Here is what Summit racing has, just click the link.
https://www.summitracing.com/search...efault&SortOrder=Ascending&tw=speedometer &sw=speedometer recalibration devices

 

And here is what they offer in programers. I have the first one. It works grate, but a hand held programer will never be as effective as a custom tune is.
https://www.summitracing.com/search/year/1997/make/chevrolet/model/c1500/part-type/computer-programmers?SortBy=Default&SortOrder=Ascending&tw=Programmer&sw=Computer Programmers

 

And here is what they offer in programers. I have the first one. It works grate, but a hand held programer will never be as effective as a custom tune is.
https://www.summitracing.com/search/year/1997/make/chevrolet/model/c1500/part-type/computer-programmers?SortBy=Default&SortOrder=Ascending&tw=Programmer&sw=Computer Programmers

Hi Ken,
Thanks for the info!

Mike

 

Hi,
I appreciate the help.
I made a mistake in my title.
My Speedodometer is off by 10%.

Anyways I took it by my local shop and they stuck a programmer on it.
They found the tire size set to 245 and changed it to 265.

I drove it and found it was still off.
I took it back and he tried LT265.
It is still low by 10%.

He took a ride with me while his programmer was plugged in.
The speedo was showing 50
His programmer showed 45 mph, the same as my GPS.

Something is wrong in my instrument cluster.
He believes it is a stepper motor.

Now I just need to find the right part.

Thanks for the help!

Mike W.

 

Or you could adjust in your head. After 2 weeks it will be second nature

 

Seriously?? Post It notes and shitty tires to make the speedo read correctly? I don't care what the speedo says if I have the tires I want. If you know how much off it is, you can adjust your brain in a matter of days. I've done this with several vehicles over the years.

 

Hi,
Thanks for the reply.
Now that I know there is an error I do adjust for it in my head.
The post it notes would be for the wife.

 

Yes, all of the instruments use stepper motors, and they are all filled with a thick oil that looses it usefullness in time. It is east to take the cluster apart and change out any of the stepper motors. There is even a fix that can be done. They can be refilled with a silicone diff oil, used in RC cars. A key sign that the oil is bad, or gone, is that the gauge needle bounces around. The gas gauge is bad about it. Here is a video of a gas gauge stepper motor repair. This guy drills a small hole in the motor, but I has seen a write up, where you fill a sirenge 1/2 up with silicone oil and put the end over the steem where the needle goes, then pull the siringe plunger to pull the air out of the motor. Once the air works its way up the sirenge, through the oil inside, then you push the plunger to push the oil into the stepper motor.