Ok. I suspected the car was going faster then it was reading, so I get it tested and here are the results....

Speedo: 25 35 45 55 65 75

True MPH: 31 42 54 66 79 92

Now for the interesting part. The speedo is the 145MPH type, the mechanic believes that the car has the wrong speedo in it, that it should have been a 85 MPH type and the only way to correct it is to put a 85 MPH gauge cluster in.

I talked to the previous owner and the car use to have a 85 MPH gauge cluster.. This seemed weird to me, so I did some research. The 85MPH speedo only came in the carbureted engines and the 145 MPH was for the TPI models.

To make it even weirder, the "original" 85MPH gauge cluster had a long needle with the MPH on the left and the KPH on the right.. the 85MPH gauges that I've seen were in the same full circle style as the 145 MPH gauge..

So, now for the question: How in the world do I figure out what really belongs in my car?? Is there a RPO code for the gauge clusters? Would putting the right cluster in fix the speedo readings?

The car is a 1987 IROC Z with a 305 TPI and automatic transmission.

 

Maybe the face plate on the speedo could just have been changed.
----------

85MPH1985 - All
1986 - All (exc. later 86 models with 305 TPI)
1987 - LG4 (305 4bbl Carb)
115MPH1988 - LO3 (305 TBI)
1989 - LO3 (305 TBI) and all convertibles, regardless of engine

145MPH1986 - Later models with 305 TPI
1987 - LB9 (305 TPI) and L98 (350 TPI), including convertibles w/ LB9
1988 - LB9 (305 TPI) and L98 (350 TPI), including convertibles w/ LB9
1989 - LB9 (305 TPI) and L98 (350 TPI), excluding convertibles
This says yours should have come with the 145 mph.

 

OK. so I was right about the fact that it needs to be a 145 speedo, now the question is... How do I correct the difference in what it reads and how fast the car is going? Someone told me that changing the gear(?) to a larger one would help it, but it wouldn't be able to correct it any more then 8 mph or so.

 

For help with the correct trans speedo gear to match your cluster, send a PM to BigBabyLou. You'll need to provide him with some info ie. tire size, rear gear ratio etc.

If the RPO sticker is still in your car, find the rear gear ratio here:

http://www.sethirdgen.org/reardata.htm

 

I'm right here.

Make sure you know your gear ratio. The previous owner might have put in different gears.

Then we can figure out what plastic gears for the speedometer pickup you'll need.

Lou