Cluster speedo not accurate

[densobro]

It shouldn't be off the same amount at each speed if speedo is working good. Example, assume it's off by 7% like if larger tires are installed:

@ 40 MPH : 40 x 1.07 = 42.8 MPH (difference is 2.8 MPH)
@ 80 MPH : 80 x 1.07 = 85.6 MPH (difference is 5.6 MPH)
...etc. it's a linear relationship

As you increase the speed, the difference should be increase also proportionally, and vice versa...

Yeah the number in the 70mph range that a gave are for sure not reality. My wife wasn’t as invested as me when giving me actual speeds and maintaining the same speed as me haha

 

[densobro]

What tires did your truck have ? On the door jam or the RPO on the glove box. Start there first. Compare to what tires you have now. 20 is quit a bit off. Start with tire size first.

Tires are p235/75r15
And this is the door jam sticker

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[Macs Garage]

Mine was off 5 mph at all speeds. I pulled the needle off and put it back on in the correct place. Now reads correct according to GPS on phone.

 

[densobro]

Mine was off 5 mph at all speeds. I pulled the needle off and put it back on in the correct place. Now reads correct according to GPS on phone.

I thought about doing this but didn’t want to make it way worse, but not that I have a second cluster it’s the perfect time

 

[kennythewelder]

Well, this is what I have, and I got it from Summit Racing several years ago. I have a 97, 5.7L with a 4L60E. The Jet programmer I have, says on the box, 1996- 2000, I think. I also know that my programmer, will tune a 96. I have recalibrate my speedo several times, and I have a 120 MPH, Denial cluster in my truck now. I had to recalibrate the tire size, to get it to read correct. Then last year, I blew up my rear end. I went back with a set of 373 gears, and an Eaton trutrack. I GPS my speedo, and did have to reset the ECM for the 373 gears. So, yeah, I have do this quite a few times.

 

[densobro]

What size tires are you running. Gearing, and tire size will affect your speedo. Here is a grate video on how to find your gear ratio, without pulling the diff cover.

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That’s pretty cool, I’m a maintenance technician and do everything like welding, electrical, mechanical and machinist stuff and I’ve used this method to determine what a gear box is geared to if the name plate is missing but I never thought to do it with the rear axle
Great video. By the way Kenny your truck looks super familiar, did you use to make YouTube video? Is that white Silverado in its fourth motor? I might be mistaken

 

[densobro]

Well, this is what I have, and I got it from Summit Racing several years ago. I have a 97, 5.7L with a 4L60E. The Jet programmer I have, says on the box, 1996- 2000, I think. I also know that my programmer, will tune a 96. I have recalibrate my speedo several times, and I have a 120 MPH, Denial cluster in my truck now. I had to recalibrate the tire size, to get it to read correct. Then last year, I blew up my rear end. I went back with a set of 373 gears, and an Eaton trutrack. I GPS my speedo, and did have to reset the ECM for the 373 gears. So, yeah, I have do this quite a few times.

That jet programmer might just be my answer, and I’ve been looking into the denial 120 cluster also, just seen a video of a guy explaining everything that need to be done to complete it. The guy was driving the truck after words and it was raining out. I really feel like your that guy haha

 

[kennythewelder]

That jet programmer might just be my answer, and I’ve been looking into the denial 120 cluster also, just seen a video of a guy explaining everything that need to be done to complete it. The guy was driving the truck after words and it was raining out. I really feel like your that guy haha

Never did any YouTube videos, but I have seen a few trucks like mine. Well, not exactly like mine, but similar. I really like the 120 cluster. I used that same method of checking my gear ratio, several years ago when, I had the OE gears, before I rebuilt my rear differential. IDK, for sure but, I think that over time, the stepper motor gets to where it just isn't as accurate any more, or it gets out of the OE calibration. One or the other. After using my Jet programmer, the speedometer has been accurate.

 

[df2x4]

Just FYI, for another $60 or so on top of what that JET handheld programmer costs you can get the JET DST software. Takes a laptop to run, but it's capable of far more customization than the handheld programmers. It's the only tuning software I'm aware of that's compatible with the '96-'97 black box PCMs in these trucks. I've been considering picking up a copy for a while now.

www.amazon.com/dp/B0018CWGS8

 

[sewlow]

This is the one I have. Not the best. Not the most expensive, but not the cheapest either.
The 'performance' gains were negligible.
What I do like about it is that can remove the speed limiter, change shift points on automatics & most importantly, adjust the speedo for different heights of tires or for gear swaps.
It corrected the speedo to be dead-on when I went from 3.42's to 3.73's.
I used it for both trucks ('97 & '98) whenever I swapped the 15's, 17's or 20's around between the two.

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