I have 70 Buick with a Muncie 4 speed the speedometer stopped working the other day I climbed under car and found this device ; however the speedo cable still works I can spin that fine; so the speedometer housing that says Stewart Warner obviously is broken cover is missing and no speedo gear See pics. my question is what housing/ part can I buy eliminating the old one and where to work in this trans so speedometer will work again Thanks in advance
The SW gear reduction device number is different for 3.42 and 3.64 rear ratios. Look for a used unit.
Mine failed the same way. Although, mine was slightly different. I replaced the plastic gear inside the "bullet" that mounts to the trans, removed the SW completely and went with a long cable. With my 3.64 is pretty close to accurate.
Is this what you have now? https://www.ebay.com/itm/AC-90-Degr...114191506616?_trksid=p2385738.m4383.l4275.c10
That's an adapter, it reduces or increases the cable speed because there is no DRIVE/DRIVEN gear combo to provide accuracy with your rear gear ratio. Are there any numbers on it, might give you a clue to the ratio. They are available. You may need to figure out which one you will need, this page will help. https://speedometercablesusa.com/gear_box_adapters.html
3.64 (or .785 ratio) should be a yellow code/dot - Part number 396338 A different number likely makes it the other one which is for a 3.42.
one other question does a unit have to go back in like i currently have or can i use a standard housing gear for speeedometer?
I'm not as familiar with the manual transmissions as I am with the Automatics, but the Automatics use a big 2" diameter sleeve for the DRIVEN gear. It looks to me like the 4 speed uses the smaller sleeve like the Chevies. That limits the selection of DRIVEN gears you can use, and that might be why an external adapter is necessary. In any case, your rear tire size and rear gear ratio determine what you need. With the Automatics, you just plug in your numbers and it tells you what you need for a DRIVEN gear. So, rear tire height, rear gear ratio, and number of teeth on the DRIVE gear (on the output shaft). https://www.tciauto.com/speedometer-gear-calculator You should be able to see the DRIVE gear through the sleeve hole, it's a helical gear, so it is harder to count the teeth, but it can be done by rotating the transmission input shaft. This is the Speedometer Gear Chart (Skylark/GS) from the 1972 Buick Chassis manual.
Something is not quite right on that chart, at least for the 3-speed and 4-speed manuals. Here is an example what the driven gear tooth count should be when on the chart when looking at G70-14 tires, only changing rear axle ratio: 3.08: 19 teeth 3.42: 21 teeth 3.73: 18 teeth As rear axle ratios increase, the output shaft on the trans turns faster at a given speed, so the driven gear to the speedometer needs to turn slower to compensate (which means have a higher tooth count). Something isn't right with this example - and it looks like there are other examples. Where did that chart come from?
1972 Buick Chassis Manual, page 40-90. That chart is right on the money for the Automatic transmissions. Probably not for the Manual. Maybe that is why Buick used the ratio adapters.
This one on ebay seems to match your housing - but I don't know if it is ratio'd differently than yours. https://www.ebay.com/itm/STEWART-WA...272045?hash=item547f7ed7ad:g:nIAAAOSwhBFeSB4s
The chart is not entirely logical for automatics either. Use the THM 350 with G78-14 tires, ratio to driven gear tooth counts are: 2.56: 35 2.73: 37 2.93: 40 3.08: 42 3.42: 38 The 3.42 rear axle ratio doesn't make sense. Unless the 3.42 ratio rear on the THM 350 w/G78-14 tires also had a ratio adapter. One would think the chart would reference the need for a ratio adapter in this case.
