Help me diagnose wrong speedo readings!

[dpok69]

I am trying to track down why i get wrong readings on my digital dash and scan master. If i travel 80mph, the car reports 40 mph, way off!

This may be affecting my gas mileage since ECM also sees this.

This is what i know:

There is a black cable near my brake resovoir with a grey box type coupling. Someone told me this is a reducer. Why would someone install this?

The previous owner had a monte carlo. The trany was crf code(maybe cz).

Just had my rear serviced (fluid change). My GN mechanic did not tell me anything was peculiar with the rear gears (but then again he was not looking for anything).

 

[MeanBuicks]

Well, if someone previously installed an incorrect transmission, they may have put the reducer on there as a band-aid. To go back to where you need to go, assuming you have OEM, 26" tall tires and 3.42 gear still, you'll need to have a blue 30-tooth driven gear and the orange 11-tooth drive gear on the governor and remove any reducer.

If you have changed the rear gear and/or tire size, look here for assistance.

 

[terryk]

I saw this thread and went to the TCI site.

If the stock 3.42 FD with stock tires (26") is 11/30 gear combination, why does the TCI site come up with a 29.152T gear? 29T would be closer to correct not 30T.

Why is stock a 30T rather than the 29T which is closer to correct?

I want to go with a 235/50-15 tire. It looks like a 31T driven gear would be about right, but considering the error above, I'm not so sure.

 

[chris718]

we can help you out if you cant find the right gears we stock most of them .www.ckperformance.com

 

[terryk]

Thanks Chris, I appreciate it. RIght now I'm trying to understand why they results don't match.

TK

But once I know what I need, I'll certainly ping you.

 

[chris718]

the speedometer is not 100 percent accurate in many applicatios unless it states on the head CERTIFIED CALIBRATION like it does on police ,fire and ambulances .also the factory tire height is not exactly 26 inches .this is also why you have a 7 mph leeway zone in most states which allow 7 over the limit above 55 mph before you are truly speeding.the box you see under the hood is a trans ducer used to change mph reading at the speedometer head when tire size doesnt jive 100 percent with the drive and driven gears available for the vehicle. it also works cruise control etc in other appklications .www. ckperformance.com

 

[terryk]

I guess I'm trying to understand why the formula doesn't agree with the actual gears (or vice-versa). The 29T is closer to correct according to the formula.

Probably not worth losing any sleep over.