Speedometer was only off by 5mph and now progressively off? Why?

So I need some help and what is causing this and how to fix it. I have a 1998 C1500 5.7L 4L60e ECSB with now 32” tires and the stock 3.42 rear gears.

So some back story, I put the larger tires on my truck and corrected the speedo through a OBD2 engine programmer. Worked just fine until I swapped my dash to a 120mph cluster. Then it was always reading 5mph fast because I was to lazy to adjust the needle (needle got swapped from the cluster).

Now something happened recently and not sure why it would cause my speedo now to be progressively fast. Like city driving it’s about 4-5mph off being under 50mph. But at highway speeds I’ll pace myself at 70 with others and gps. It will say I’m going 85 and gets worse at 80. Any ideas what is going on and what I can do to fix it?




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Sounds like the programmer isn't correct.
Going with larger diameter tires, actually slow down the speedo.
Something in the electronics is way off.

So here’s the kicker I haven’t played with my program in probably 3 years. Now this means I had a corrected speed read out. So if I was going 70 I really was going 70 with the larger tires on. So once I swapped in the 120 mph cluster which is not factory to my truck the needle is 5mph faster. So because it was off by 5mph I couldn’t edit that out in the programmer because tire size didn’t change or anything, it’s a needle position issue. This is probably because the needle was moved at some point from the person I bought it from. But now when I drive it the speed is progressively off. So it has nothing to do with the programmer since I haven’t changed anything on it since the original cluster. Also the programmer does not stay plugged in. What is going bad?


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How does the tach look? Is it reading higher now too?.
My speedo is off by 33% and my tach is off by 2000rpm. I suspect the cluster has gone bad. I just haven't had time (or found a spare) to swap the cluster to check.

All the gauges including the tach are good. All the normal operating temps are correct and the RPMs read correctly.


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I do not know anything about the GMT400 speedo set up. I have played with several 2nd gen F-body & a few Vega speedos .

It's possible that the current 120 speedo is geared differently than the original 100. I know there's electronics involved with the GMT400 stuff, so I can't say exactly what is going on. But there has to be gears involved to rotate cables, sensors, transducers, etc.

I know gears are progressively off. If the gear is 10% off, the resulting output will be 10% off.
0 mph = 0 mph
5 mph = 5.5 mph
50 mph = 55 mph
500 mph = 550 mph

The difference (or error) gets progressively higher (or lower) as speeds change.

The other issue .... Needle changes, by its self, do not have progressive errors. If needle is 5 mph off, at 0 mph speedo will read 5 mph.
Easy to see needle misplacement errors at 0 mph.... lol. ... which do not change progressively.
5 mph = 10
10=15
20=25
100=105
etc....

Since you have progressive issues, I'd venture to say your issue is speedo system gearing, not needle placement.

Probably have to change the transmission speedo output gear to match the current 120 speedo.

In the systems I'm familiar with, the transmission output shaft turns a gear that is geared to match the mechanicals of the speedo.

If you change the rearend gear ratio, the transmission or the speedo, that gearing relationship is changed between all 3 devices .... resulting in a progressive error like mentioned above.

Typically, transmission speedo output gearing sets are color coded to match rear end gear changes. In example: 3.08 rear gear gets one color speedo gear, 3.42 gets another color, and so on.....

Changing the speedo to a 120 unit, probably requires a transmission speedo output gear color change.
........ because the travel sweep of the needle (distance the needle travels) did not change (actual distance traveled from 0 to 100 and now 0 to 120 stayed the same).
So, if you go 100 mph, your new speedo should say 120 ..... you didn't change the travel sweep distance.
The 1 mph increment distance changed, not the needle distance (say 0 to 50 was 5 inches, now its 4.5 inches).
That requires some gearing change to compensate.

So I do agree that the gearing could be different in the 120mph vs the 100mph.

This is the catch... nothing has mechanically change from it being 5mph to now being progressively off.

What I am trying to say is I didn’t change tire size, rear end gearing, trans, nothing. Something is going bad and not sure what that would be.

-Went from stock cluster reading 100% accurate on big tires.

-100mph cluster to 120mph cluster reading 5 mph fast from 0 to 120. (50=50 & 100=100).

-Current day having the speedo be progressively off. (40=35 & 70=85)

Between those 3 stages nothing mechanical has change on the truck. That’s what I’m trying to say. It wasn’t progressively off when I first swapped in the 120 cluster.


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bugdewde said:

In the systems I'm familiar with, the transmission output shaft turns a gear that is geared to match the mechanicals of the speedo.

Click to expand...

In the GMT400 platform there is no speedometer cable, in 88 GM started using VSS (Vehicle Speed Sensor) where a reluctor in the tail shaft senses the metal gear points spinning and sends pulses to the DRAC (Digital Ratio Adapter Controller) or VSSB (Vehicle Speed Sensor Buffer) in earlier years (88 - 95), usually ~2000 PPM (Pulses Per Mile) which then sent signals to the ECM/PCM and speedometer. In the later years (96+) it used 4000 PPM (IIRC) fed directly to the PCM which then sent the signal to the speedometer. When you calibrate for different size tires or gear ratios you're adjusting the PPM up or down in your base calibration (jumpers for 88-95 and programming for 96+). That shouldn't change without program adjustments.

matad311 said:

Between those 3 stages nothing mechanical has change on the truck. That’s what I’m trying to say. It wasn’t progressively off when I first swapped in the 120 cluster.

Click to expand...

Sounds to me like your stepper motor in the cluster is going out.

PlayingWithTBI .... thanks for the explanation.

With that info, I would agree that the stepper motor inside the speedo (speedo gearing in my old school vehicles) is the factor that has changed.
It is probably going bad.

Swap the original back in to comfirm.