Is it possible? The mechanics around here are shady. I've called two so far. The first guy said it's not possible to recalibrate it. The second guy said he'd charge me $74 an hour to look at it.

I live in Central Illinois. Any help would be appreciated. The Speedo is 20MPH off. I just bought the car. When I'm doing 55, I'm doing 75.

Thank you,
Matt

 

The speedo gears are in the tailshaft of the transmission. You need to find out what your rear end gears are, and then get the appropriate speedo gears to match.

Has the rear end or tranny been changed from stock?

 

I'm not sure. I've sent an email to the seller to find out if he can find out for me what gears are in the rear end. If he can't, I'll probably just have someone pace me and mark the appropriate speeds on the speedometer with a marker or stickie tags. It'd probably be cheaper than taking it in somewhere anyways. If he tells me what gears are in there, I'll reply back with that.

Thank you

 

put the car on jackstands.
put the car in nutural.
get under the car and note the angle of the driveshaft.. its easiest if you turn the yolk vertical or horazontal to start with.
have a buddy turn the tire forward exactly one revolution slowly.. or rotate the driveshaft and have your buddy watch.
if it turns 2 and 3 quarters,(2.75ish) you probly have 2.73s.
if it turns just under 3 and a half,(3.5ish) you probly have 3.42s.
ect...
this will get you a reasonably close idea of what gears you have in there.

once you know this, you can lookup on here what tailshaft gears you need.


btw, just so you know, its off a PERCENTAGE... not a MPH number.

 

After a gear change from 2.73 to 4.10 in my 92 RS the speedo was off about 33%. The "quick" fix is an ratio adapter between the transmission and speedo.They make them for electric and mechanical applications.The little mechanical guy is about $60 and the electronic ratio adapter can be found for between $130 and $150. This link has a pic of both...just scroll down for the GM ratio adapters.
speedometer_calibration_______va

 

"just talked to the guy I got the car from, He told me he never changed the gears . I said wrong spedo gear earlier , but that car should use speed sensors the speedometer is elecrial not cable controlled .Look on the carfax report that I gave you w/ the car it may tell you what option pacage the car has(rear end gears, transmission,OEM tire size) If not take the car or the vin to a pontiac dealer and they should be able to break down the vin and tell you what options the car has , what transmission speed sensor it should have , and other possibe causes for this problem.Hope this helps."

The above is the email I got from the guy I bought the car from. I'm not too mechanically inclined. I appreciate the tips everyone has given me on here, as I can print it out and take it to a mechanic.
So, since the speedometer is electronic, would I just need to buy the $149.00,

"Electronic Ratio Adapter (ERA)

 

ok, heres how it works, so you can make your own decision on what to do:


the tailshaft of the transmission, where the driveshaft connects, has a little plastic gear on it. it turns however fast the driveshaft (and therefore the wheels) are turning.

90* to this plastic gear, is the VSS (vehicle speed sensor) and it has another plastic gear on it. this gear slides on and off easily... and the two plastic gears are replaceable.

the reason they are replaceable, is they're ment to spin the VSS at a specific speed.

the VSS is supposed to put out a signal of 4000 pulses per mile.
this then goes out to the ECM, speedo, and cruise control.



thats the basics of it.
you have two choices:


you can modify how fast you spin the VSS... aka replace the plastic gears. this is the GM method... replacing the gear on the VSS is NOT hard.. its only one bolt, slips on and off... really easy actually. i did a writeup years ago.... post number 8 here:
replacing the other gear is a bit harder, but still not impossible for the avg guy. it is kinda messy though.

or.

you get a signal box.
this box will take your VSS as a input... then modify it and spit out a different output.... if you set it right, it will put out a accurate 4000ppm signal, and fix the problem.





of course, all this is assuming that this IS the problem, and yoru speedo isnt broken.. lol.