Th 350 speedo help needed

I need a little help here with speedo gears. My current speedo is off about 25 MPH (fast). How or What do you change to get it right. I have a speedo reducer on the tranny now (Between cable and case) but not much help....

Spoons

 

pull the housing and plastic gear out of the trans and count the teeth. go to you local gm parts counter and order 2 or 3 with more teeth than what you have and try them they are fairly cheep so get an assortment...

 

The rule of thumb I've always used is 3 teeth per 5mph off. So in your case you need 15 more teeth on the speedo gear in the trans. With that much off you may have to change the gear on the output shaft also.

 

I have always used 1 tooth for 5 mph at 50-55 mph. And yes,you may need to change the drive gear on the output shaft

 

Here's a link to a short tech article on this subject:

http://www.classicfirebird.com/hand/jhand16.html

 

heh....there are two different styles of driven gears used on the th350 with quite different tooth counts, and there are at least three different drive gears for the small Chevy ones and at least two for the bigger BOP setup.

and speedo reducer boxes come in an almost infinite variety of ratios...but you can swap out the gears in them to make them do what you want, if you have enough of them to mess with.

Also, saying it's 25 mph off doesn't help too much if we don't know how fast you're driving when it's off that far. 50? 75? makes a big difference.

to find out how much you need to change the gears or adapter box to make it work right, find a marked mile stretch of road and see how far the odometer turns in one measured mile. Use the percentage it's off to caclulate how much change you need. Then look at the driven gear, if it's at the low end of the range you can probably just change it and solve the problem. if it's not, then you'll need to either change the drive gear (requires pulling the driveshaft, mount, tailhousing, and the gear), or else get an adapter box made or make one up yourself, or maybe you'll need to do all of these things.

 

just go here:

http://www.tciauto.com/Products/TechInfo/speedo_gears.asp