Generation Specific => Second Generation 1970-1981 => Topic started by: Tone on March 16, 2023, 08:44:53 AM
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Good Morning, I have a 1971 Trans Am 4 Speed with what I believe to be a 342 gear... Can anyone tell me the proper speedometer gear to use? Thanks...
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What diameter rear tyres? Also, pointless only believing. You need to know.
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TA Radials 255/60R15... No idea as to diameter...
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Righto.....google says 27.1".
Here's the chart for original tyres and ratios which you can compare to. Originals were close to 25.9" diameter so will not be accurate.
(https://live.staticflickr.com/65535/52753153845_ddf0ae9794_k.jpg) (https://flic.kr/p/2onBGKg)Firebird 1970-72 4-speed speedo gears (https://flic.kr/p/2onBGKg) by Ben (https://www.flickr.com/photos/196419001@N03/), on Flickr
I'm not familiar with early Muncies, were they still using an 8-tooth drive gear for all?
Based on that, and a certain amount of guessing, you'd want closer to a 20-tooth instead of what is possibly a 22-tooth driven gear.
https://www.allstategear.com/Muncie-Speedo-Gears-s/2122.htm
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255/60 - 15
255mm x 0.60 = 153 mm
153 mm Divided by 25.4 cm per inch gives 6.02" the height of the tire.
15" rim + (2 x 6.02) = 27.04"
Next step find the rear-end gear ratio. Like Wallington said, pointless if you don't know.
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It takes some time to calculate, but this page shows the actual methodology used by carmakers to calculate speedo gear. There is some factored in inaccuracy that will happen no matter what, the point is to minimize it:
http://429mustangcougarinfo.50megs.com/speedo_gears.htm (http://429mustangcougarinfo.50megs.com/speedo_gears.htm)
The page is helpful if you have some sort of modification that would alter the speedometer calibration, in that you can "recalculate" from scratch with your modifications. However, if you are BONE stock: same diameter tires as originally equipped, same rear end gears, and same trans gears, then you can probably get the right speedo gear out of a service manual.
My '79 W72 4-Speed has only one change that would affect what speedo gear to use, and that's slightly wider tires - diameter is just about the same as the factory Polysteel radials, but they're just slightly wider BFG Radial T/A's. After doing the calculations, turns out the factory gray 18 tooth speedo gear that I already use is still the best one.
Don't forget to factor in if you use a speedo gear reducer/adapter - usually installed in the middle of a two-piece "upper" and "lower" speedo cable. I don't think this is likely in the earlier 70's cars, but my '79 came equipped with one. The reducers were usually produced by Stewart Warner.