This one has been bothering me for a while.

- The car is an 83 Z28 HO with 3.73 (GT4) rear end.
- My speedo is inaccurate, reading about 10 mph less than what I am doing.
- It seems to be constant and not exponential.
- The needle jumps a bit at low speed.
- This car does have cruise control, so the speedo cable is going to that, then up to the dash with the second cable.

1) I have confirmed that the transmission is original.
2) The rear end also seems to be original.
3) I am running at 2300-2500 rpm at 65 on the highway which seems to be correct, and rules out other ratios.

I'm looking for reasons that this can be off that do not have to do with swaps, and rather part failure. I find it very hard to believe that anything is not original at this point.

I was thinking that maybe the driven gear is damaged somehow. I had to tighten the speedo cable to stop it from leaking. Has anyone seen this? It would probably be the first place that I dig in.

I was thinking that maybe the cruise control could have some effect on it. Does anyone know how the gears mesh? Issues?

Or lastly I was thinking that maybe the gauge assembly has an issue itself.

Anyone deal with this before without a drivetrain swap situation going on?

 

Those don't make sense together because you left something out. What's your control? According to my imaginary friend, my speedo is 10mph off?

I'll assume you're a GPS (app) owner and entrust speed verification to it. Yay.

If you have a stretch of highway and in 15 mile markers, the odometer ticks off 14.9 to 15.1 miles, then the speedo gears, cable, and inner geared operation of the speedo are not the issue. The issue is with the spinning magnet / speed cup of the speedo and / or a mis-installed speedo needle.

These parts may still be available for your (dual-needle?) speedo or NOS somewhere. Or getting the two relevant parts from a donor speedo may be an option. I've had luck with both methods. Manufacturers often build different speedo faces onto nearly identical mechanisms.

 

Thanks for the reply,

When I first got the car I was driving alongside my wife and called her when I noticed the speed being off to figure out where I actually was. I got my bearings then so that I would not speed on my way home. I also pay attention to the many radars that are set up alongside the roads that tell you how fast you are going and compare them to the speedo, so I have done this at a lot of different speeds. As far as knowing what the RPMs should be, I entered the rear end, transmission, tire size into online calculators to see what the RPM ranges are for 5th gear.

I'm going to use the mile markers as you say to rule out the cable side. It is a dual needle speedo. I am not familiar with the workings so you have helped me out in knowing that there may be a fault in there with a couple of components.

 

Did you car come stock with the T-5?

 

Yes, the whole drivetrain is stock.

To update, the other day I was driving and paying attention to the mile markers. I was only on the freeway for about 5 miles, but it seemed as if the speedo was counting the miles right on the dot. I will do this again with a longer stretch, but it is looking as if it is an issue with the needle itself somehow.

There is a car at the local U-pull that still has a speedo installed. I can probably use it for parts if necessary.