Speedometer off by 20mph.

Like I said above find out your transmission also by counting the rotations and come back and post and I can tell you which transmission you have because of your 1st gear ratio. <trbkrp> try staying on task I know what he wanted but the speedometer gear inside is also based if this is a 4cyl or 8cyl trans.

im having the same problem.. i got a 90 gt 5.0 with the 373 and im about 10/12 mph off.. but i dont care as long as it says im goin the speed limit i know im doing 10 under and im not getting pulled over ..

A burned cable is not going to make it read 20 MPH faster. It would have to magically spin the cable faster to do that. I would replace it anyway since it will likely fail soon. I've burned 2 or 3 over the years. The last cable I got must be longer since I was able to route it far away from the exhaust and haven't had any problems in a while.

But first you do need to figure out what the rear gear ratio is and then what tranny you have, specifically what speedo drive gear is in the tranny. Check the rear gear like was stated above, and then see what gear is in the tranny, and then we should be able to find you the closest speedo cable gear. If you can't see in the speedo gear opening, you can try looking for the tranny tag info (most likely long gone if it was rebuilt) or like speedy said, count the input shaft vs drive shaft rotations to attempt to identify the tranny, but that won't tell you for sure what speedo drive gear is on the output shaft.

I have 3.73 gears with a 7 tooth drive gear in the tranny, and my speedo is still about 4 mph fast at 60, but that is a lot better than being 20 MPH fast.

Do the checks and get back to us and we can get it stratght for you.

The stock range of speedo gears work correctly with a limited range of tire sizes. The 23 tooth white gear works good for some guys, but self destructs in a few months for others.

See http://www.corral.net/tech/gearcalc.html for more help

Check to see if there is a speedometer shop in your area. If there is, ask about the Stewart-Warner speedometer gearbox. I believe it is either a 777 Series Drive Joint Kits and Parts or 666 Series Drive Joint Kits and Parts. It is a small gear box that fits between the speedo pickup gear on the transmission and the speedometer. It has quick change gears that allow you to choose almost any tire size and rear end gear ratio you want. This will allow you to get the accuracy with within 1%-3%. The drawback is that it isn't cheap.
Please excuse the typo (their's, not mine) in the following picture - it should read Custom ratio adapters, not Custom radio adapters.

See http://www.gaugeguys.com/ratioadapt.htm for more info.

1FatPony said:

I spun the tires and I was pretty exact and it came out to be 3.5 which I am assuming is 3.55 gears. If my drive and driven gears are stock, should I expect a 30% increase in my speedometer??

Click to expand...

Yes, that's about what you would expect.