# 66 speedometer off by 20 mph



## gjones (Oct 8, 2012)

Recently switched out 326 w/st300, installed 455 w/th400. Speedometer is off around 20 mph. I go 40-45, follower said it was 60-65. Haven't pulled gear out yet. Car originally had 14" stock tires. Now has 245 60 r15 's. Any quick way to calculate gear ratio? Also, are there any special instructions for this particular transmission (th400) when pulling the gear? Thanks


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## BearGFR (Aug 25, 2008)

If you've got a smart phone with a good gps function, use it to read your actual speed and compare to indicated speed. Once you know that, there are web sites out there to help you calculate what kind of speedometer gear change you need. The only tricky thing I know about on the TH400 is that there are two different speedo gear housings based on number of driven gear teeth. There's one housing for the higher tooth count gears and one for the lower tooth count gears. Depending on what you have and what you need to go to, you might have to change housings too.

Bear


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## gjones (Oct 8, 2012)

*speedometer gear*

thank you Bear for your response. After what I've gone through so far, it doesn't sound like I want to pull the transmission tail housing apart (she has a short nosed tail housing) So, hopefully I can just pull the gear out and put the lowest ratio gear in there (if possible) to up the mph at the speedometer- if it's close, I'll live with it- 20 mph off is just too much as it stands.


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## jmt455 (Mar 26, 2012)

A lot of good info here and some handy calculators for speedo gear selection.
You will need to know what the axle ratio is and what speedometer drive gear is in the transmission.

Speedometer Gear Calculator and Charts - TCI® Auto

I think the easiest way to determine the axle ratio (without pulling the diff cover) is to jack up the rear end (both wheels, if it's a posi unit) and count the number of drive shaft revolutions it takes to turn the rear wheels exactly one revolution. The number of drive shaft rotations that corresponds to one full tire revolution is the rear axle ratio.

Be safe - Use good jackstands!

Car must be in neutral in order to be able to turn the drive shaft by hand.


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## geeteeohguy (Feb 2, 2008)

No need to pull the tailhousing apart to change the gear housing. I went thru this when I swapped my 3.36 rear end for a 2.56. The new gear housing is held on by a clip and one bolt....a 5 minute job, but have a drain pan handy. I got the proper gear and housing from some outfit in Florida, for cheap. I think the place was TCI? The pressed gear on the tailshaft of the trans is left alone. No need to worry about it. Like Bear said, get on line and start doing the math.....that's the biggest time-consumer, but for under $30, you'll have a calibrated and accurate speedometer.


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## BearGFR (Aug 25, 2008)

Oh, and by the way - the housing I'm talking about isn't the tail housing - it's the housing that the driven speedo gear rides in - the same housing you have to pull anyway to get the gear out. Changing it is no big deal at all. On the side of the trans where the speedo cable connects, there's one bolt holding a spring clip that holds the speedo driven gear housing and gear in. Remove the bolt and the clip, and the housing will slide out. (Have a drain pan handy because it will also puke a little trans fluid when you do). Work it out gently, there's an o-ring seal around the perimeter of the speedo gear housing and you don't want to damage the o-ring. If you're re-using it, lubricate it with some tranny fluid before re-installing it.

Here's what the housing looks like:
TH400 Speedo Gear Housing WITH EXTRA SEALS 40-45 Tooth FREE WW SHIP Speedometer on eBay!

You can get pretty close without having to know your rear gear ratio or tire diameter if you use the percentage method.

Use your gps on your smartphone to get an actual speed for a given indicated speed. Accuracy will improve if you do it at a faster speed... for example, I think you said an indicated 40 was actually 60, so go out on the highway and as close as you can, nail the speedo exactly on 45 and see what the gps says. Just for an example, let's say an indicated 45 is actually 60.

Use this formula:
[ (actual - indicated) / indicated ] * 100

[(60 - 45) / 45 ] * 100 = 33.3 --- your speedo reading is off by 33.3%, and it's too LOW (if it was too high, the percentage would come out negative)

Ok, now pull your speedo housing and count the number of teeth on the driven gear. TH400 driven gears are also color coded so you can double-check yourself by color. Let's say your driven gear has 43 teeth - the purple gear. 

Since your speedo is reading 33.3% low, we need to turn the gear 33.3% faster, so that means we need 33.3% FEWER teeth on the driven gear. 43 - (33.3% of 43) = 43 - 14.3 = 28.7 - call it 29 teeth. 

BTW, being off 33% is a -ton-, if you're going to go this way please make sure you do the GPS check very carefully and get as accurate as you can on both the indicated and actual speeds. Even as little as 1 mph can change the results. That's why I said buy the calculated gear plus the ones on 'either side' of it.

If you find that your trans currently has a gear with 40 teeth or more, and the gear you need has less than 40 teeth, you're also going to have to get "the other" gear housing. The housing dividing line on TH400's is 40 teeth and up, 39 teeth and below. If your speedo really is off 33% though, you could find yourself in a situation where correcting it will require changing BOTH the drive gear and the driven gear because the driven gear you need isn't available and you'll have to get the percentage change through a combination of a drive gear with MORE teeth and a driven gear with FEWER teeth. If that happens, the tail shaft does have to come off to change the drive gear. The drive gears also are color coded by tooth count.

There's a color code chart on this page:
Speedometer Gear Calculator and Charts - TCI® Auto

If that turns out to be the case ( you need both gears ) then an alternative would be to install a speedo ratio correction gearbox like one of these dudes:
http://www.texasindustrialelectric.com/speedo.asp

Bear


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## geeteeohguy (Feb 2, 2008)

Exactly. Mine came with a 43 tooth gear with the 3.36 ap, and when I installed a 2.56 gear, a 34 tooth gear (and gear housing) did the trick. I cheated, though, and used a friend's copy of the 1967 GM manual that had all the gear colors/tooth counts posted. No math for me!! But, I am running stock diameter tires. Again, the "figuring out" is the biggest time-burner here, not the actual purchase and install of the new gearset.


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## gjones (Oct 8, 2012)

*speedometer gear*

Woo-Hoo! Thanks, guys. I kind of went through this with the st300- the clip inside holding the drive gear was broken, and incredibly, I found both pieces fishing inside the hole. With a fabricated tool, I was able to push the gear back, install the clip, and pull it back onto the clip. The speedo. was slightly off with the 15" tires, but wasn't by much. I was afraid of any different procedures with the th400. (as a side note- I've actually changed clutch packs and installed shift kits in Torque flight 727's, and such. BUT-that was 25+ years ago). I don't know why I'm so unsure of myself lately. I guess I don't want create more problems for myself. Better safe than sorry, I guess. THANKS!!!!!


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