turbo 400 speedo gear

[ken ruhland]

I installed a 350 chevy with a turbo 400 trans and wondering what speedometer gear i need to make it work. Does it require a reducer or can i just change the drive gear in the trans? Thanks

[randyscycle]

Running a TH400 in front of my NP200 stock transfer case, and connecting the speedo cable to the transmission, resulted in a speedometer reading off the scale at 45 MPH actual speed measured with GPS. If I remember right the needle was pointing at the "M" in "miles per hour" at the bottom of the speedo face.

I don't think I could have gotten any gear to correct that much error.

[compexp]

you can get electronic speedos and calibrate it yourself - push button at start of one mile, push again at the end.

that's what I will do on my current build - drives me nuts (short trip) that my 725 speedo is off.

[dahoyle]

you can most likely get a gear for the TH400 which will work, but you have to set some guidelines first. How about a few details?

What matters is tire size and gearing, which you neglected to post. Even then, you need to know what RPM your speedo is set for, and I'm not sure. someone might chime in with the info. Most everything since the mid '60s has been 1000 revolutions per mile, but I'm not sure about the M715.

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

is about the most in depth source of info about speedo cal and parts I know of.

[dahoyle]

After crunching some numbers, assuming that the speedo is 1000 revolutions per mile, then you would have to run at least 42" tires, with the stock gearing, to be able to get a set of speedo gears which would read accurately.

Of course, that is based on a lot of assumption, which may or may not prove to be true.

[randyscycle]

After crunching some numbers, assuming that the speedo is 1000 revolutions per mile,

I'm gonna say probably not 1000 RPM per mile with the stock military speedo. Again, mine was so far off, that even changing the drive gear along with the driven gear, might not garner any results that would effect a usable speedo, when driven off the TH400 itself, and not the transfer case. I didn't try swapping any gears, but the last time I played with TH400 gears, there were only about 3 or 4 gears available and they went by color. Granted, this was about 15 years ago.


I know that the NP200 driven gear is tiny in comparison to the big plastic gear in the TH400.

[brute4c]

According to the parts manual, the stock NP200 speedometer pinion gear has 17 teeth...the GM ones, typically from what I have seen at the scrap yard, probably are around double that or more.

[randyscycle]

According to the parts manual, the stock NP200 speedometer pinion gear has 17 teeth...the GM ones, typically from what I have seen at the scrap yard, probably are around double that or more.

32 or 34 teeth, seems to stick in my mind for a TH400 driven gear.

[dahoyle]

Knowing the tooth count of one of the gears, really means nothing.

You have to know the tooth count on the drive gear, which is a worm cut, and the driven gear, before you can make a comparison.

In the case of the GM, as indicated in the link I provided to eliminate guesswork as much as possible, there are driven gears from 34-45 teeth, and there are drive gears from 15 to 21.

[Swampthang]

Why are you running it off the tranny and not the transfer case?