[360Rocket]

Whats up gang, got a speedo cable and gear question.

I have a 1977 Firebird Trans Am running a 29.50 x 18.50 x 15 tire and 3:90 gears.
I also have an Autometer Pro Comp speedometer installed in the dash(mechanical)
I have a stock 4 speed transmission. pretty sure the cable is on the drivers side of the tranny.

I need to know where I can get a speedo cable that attaches to the stock transmission AND screws into the back of the Autometer gauge(not clip on like stock gauge) and also
the paet # and/or the color of the gear needed to put in the transmission so I can get as close to an accurate speed reading on the gauge as possible.

and most importantly where I can buy this stuff. Hopefully a vendor on this site.

Thanks in advance for any help you can provide.

[rexs73gto]

As for the cable you my be able to get an adapter for the back of your speedo from a speed parts dealer like Summit , but as far as the speedo gear your going to have to buy the speedo adapter reduction part to get to the correct cable speed you need . The 3.90 gears that were run in the GTO's back when you could order big gears also came with a reduction adapter to make the cable run the speed that it needed to be. If you go on ebay you'll find them for sale in the trans section every once in a while. It's an adapter that screws on befor the cable & then the cable screws on to that adapter to make the cable go the correct speed. There are 2 types for ssale ones that speed up the cable & ones that slow it down. They cam in speeds of 1.2 1.4 1.5 to go up & .9, .8 , .7, & so to go down. You'll need to know whats in your trans now to know which way you'll need to go. Also you can swap out the gear in side the trans to to change the cable speed to. You'll be able to get the gears that go into the trans at any GM dealer , you just need to know what number of teeth you'l need. You can usually go to any trans repair shop & they can help you out with the gears to as to know what you may need to get your speedo correct for your 3.90 rear gears.

[360Rocket]

Is it possible that the 29.5" tire can offset some of the 3.90 gear? I found a local speedo cable shop that can make me a cable that adapts to the Autometer gauge but the reducer he has is like $125 in hand, not even installed.

[rexs73gto]

If your talking about the speed reduction unit that slows down the speed so it reads correctly then the price of $ 125.00 is not out of sight. Most of the reduction ubits that sell on ebay or even at swap meets usually go for about that or more. The reason is there not that common anymore & when you do find them they go for a bunch. So if your getting it for $ 125.00 that price is right in line & as far as installing them it usually takes about maybe 1/2 hour if you have to do them without a hoist. All they do is screw into the trans where the speedo cable goes & then you screw the cable into it. Very simple. You just have to make sure that it reads correctly after you put it in. There are a lot of different one , but if he's a speedo shop he should be able to get one that is close. Just make sure that it reads correct from 55 to 80 as if you geet one that is wrong it may gain speed as you go faster because of geometric progression. Meaning the faster you go the more the speedo will be off either slow or fast. So once it's on make sure it reads correct through those speeds as these are the speeds that you will be driving on the highway so you want to make sure it's correct at those speeds just in case , so you don't end up getting a ticket because you think your going the correct speed & thats what it shows on your speedo & your really going much faster. I have 3:73's in my 73 GTO & have a speedo gear in mine thats close but when I'm going 60 on my speedo I'm really going 72 mph. I checked mine with my GPS unit & try when ever I go any distance to take it with me so I know how fast I'm going by my GPS which is much more accurate then the speedo. If you spend the mony just make sure it's correct so you get your moneys worth.

[FrankieT/A]

For the cable, I would compare the size on the back of the speedo to a lower cable for a trans am with cruise control. Those thread on both ends and the cruise end is smaller than the trans end. Just a thought.

[Skip Fix]

Find the length and then go look through the pioneer catalog for the correct ends you need.. I converted to an Autometer in the Camaro and did that vs using the screw on adapter.

[360Rocket]

Cable is out of the car, single cable 67.5" long.

[6TEE8GTO]

I have 3.90's and 28" tires and with a 42-tooth driven gear I was off quite a bit, got the correction box on this page for $62.00 and now my speedo is almost dead-on correct:

http://www.transmissioncenter.net/sp...n_______va.htm

Scroll down for the adapter, takes all of 10 minutes to install...