marcus88

I bought a 98 z28 but noticed the speedo is way off.. At 60mph (speedo) actually 45mph @ 1500 rpm... Does this seem to be a gear a change? I want to get the pcm tuned but not sure of what is needed.. Any help would be appreciated..

97camaroguy

iTrader: (5)

rim size/tire size and a gear change could lead to the speedo being off. Try contacting the previous owner and ask them what gear they had installed.

maxharvard

Gears could be the problem as could tires.

You could also have a bad VSS.

Atrus_SS

iTrader: (1)

You can figure out the gearing. Jack the vehicle up, turn the wheel 1 revolution, and see how many revolutions the driveshaft spins. If it's different than what the car rolled of the factory with, that could be it.

As someone mentioned, improper tire size can also throw off a speedo.

marcus88

Thanks for the responses, the car has the original wheels as well as the correct tire size (245/50/16)...

marcus88

Would the vss throw a code? If so I do not have a sel light..

maxharvard

Not typically, but it's cheap so I say try that first.

1BurnedZ28

Don't throw a VSS at the car without measuring your gear ratio which is in all likeliness what is causing your inaccurate cluster reading. VSS failures will typically (and i Do mean 99 percent of the time) cause the speedo to either be inoperable altogether OR incredibly erratic (Bouncing to 140mph to 0 to 75 etc). You WOULD also get a VSS code.
I'm constantly amazed by the "Throw parts at it" mentality so commonly found online. In no way shape or form would throwing a VSS at the car make the list of the first 5 things any GM tech would even do to diagnose this problem.