I recently bought a 91 Z 28. All stock at this point. My speedo reads about 10 mph low all the time. The tach reads about 1000 high all the time. I read through all the posts here and was sure I needed a tach and speedo calibration. I took the cluster to a local shop that does GM dealer work and they checked the cluster and said it was dead on accurate; my problem was somewhere else. The wheels and tires are oem size, still has the original rear too. Local transmission shop says it would not be the VSS (speed) sensor. I'm not sure where to go from here, any ideas? Thanks!

 

Looks like the dealer is lying to you. If the speedo reads 10mph low constantly, it's not a VSS gearing problem.
And if the tach reads 1000rpm high constantly, again, it's not the coil or wiring problem.
I'm surprised that the stealership didn't recomment that you need a new cluster for $500.

Anyway, what did you compare the instruments to? GPS? A second tach?

Lou

 

I agree. At first I thought the electronics place was wrong, but, since they didn't charge me a dime for checking it and turned down the $245 to calibrate it I believed them. I still may get a second opinion, but for now I am assuming the cluster is accurate.
I took the cluster to a GM dealer yesterday and they verified it was the original cluster for the car; so I can rule out a incorrect cluster.
Yes, I checked the speedo w/ a GPS. I have not checked the tach yet (although it's pretty obvious that it is not reading anywhere close); any idea where I would connect a aftermarket tach to check the actual reading? (I have one, just not sure where to temporarily pick up the tach signal I need)
Thanks

 

not sure here...but I think the coil could cause a tach problem. module too maybe?

 

If the engine is running right and the tach isn't "bouncing" all over the place, the coil and wiring should be fine.

You can tap into the white wire that comes from the coil for tach signal pickup. I have an old (but precise) aftermarket tach that I bough a long time ago. It's a good thing to have to check rpm on any engine.

So if your speedometer always reads 10mph more than the GPS (at any speed), it's not the VSS or wiring, it's a gauge problem. You could easily swap the instrument cluster to verify that. Same goes for the tach.

Lou

 

i do have atach and now that i know what wire I'll try that and let you know what I find. Thanks for the info.