Speedo Gear Tooth Difference

[cwilkie]

I am still fighting with my truck. Now I am working on getting the speedometer accurate. I have a 45 Tooth Driven gear and at 65mph the speedometer in my truck reads 53mpg. I put another driven gear in (didn't count it, but I assume it had less teeth) and at 65mph the speedometer was reading 57mph. I have heard that for every tooth difference there is a 5mph change in speed at the speedometer. Is this theory right? My Fiance` was nice enough to help me out with the acutal speed. Please help a guy out. Thank!!

Chance

[skokie]

There are 2 gears that you have to work with. The speedo gear and the gear inside the tranny. What you can do is this. Find out what gear is inside the tranny and go from there. Also here are some links that can help you out. And a thread from another guy that went through the same thing you did.

http://67-72chevytrucks.com/vboard/s...d.php?t=204866

http://www.bgsoflex.com/speedo1.html
http://www.tciauto.com/components/speedometer_gears.htm
http://www.transmissioncenter.net/sp...n_______va.htm

[jacobs]

There are speedometer adapters available that mount between the transmission and the speedometer cable. They have three small gears inside. Many pickups and trucks came from the factory with these adapters. I know speedometer shops still offer them and all you have to do is tell them what ratio you need. The last time I checked, they had a price tag of about $75.00 on them. I think they are a little pricy for what you get but it's a quick and easy way to correct the speedometer.

[Green Machine]

You can go as small as 34 teeth, but the 34-39 require a different driven gear housing, the housing you have supports 40-45 teeth. If the 34 tooth isn't enough then you have to change the driven gear to a larger tooth count.

Best to find a online calculator, you can get it right by trial and error, but it is better and cheaperto get the tooth count right the first time if possible.

[cwilkie]

Here's what I have at an actual speed of 65mph...

With a 45T driven gear the odometer read 53mph.
With a 42T driven gear the odometer read 57mph.

I am thinking I need a 38T gear that requires another housing. I call the transmission shop where I purchased it from with my assumption and they agreed. It was also nice that they are gonna mail me what I need. So hopefully they come through.

[JAKES 68GMC]

I wish my truck could go 65mph.