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Thread: correct speedo gears for 4.78 final drive

  1. #1
    Join Date
    Aug 2005
    Location
    Richmond, BC
    Posts
    132

    correct speedo gears for 4.78 final drive

    I recently upgraded to 4.78 final drive. With the 4.78 and stock speedo gears (19:5), the speedometer would be off by 10%. After a bit of research, I found the part numbers of the correct speedo gears. So I gave it a try and have confirmed that they would work with the T-50 tranny.

    Plastic gear p/n: 33403-19155 (N=20)
    Metal gear p/n: 33481-12030 (N=5)

    Marvin

    *** Important update 2008/09/03 ***

    The 4.78 final drive and 20:5 speedo gears have been installed for 2 months now, and I started noticing that the indicated speed is somewhat higher than the actual speed, at least more than the 2.7% I anticipated. So I borrowed a GPS, and to my surprise, found out that the indicated speed is 10% higher!

    Previously I calculated the speedo gear ratio to be 4.11. Between 20:5 and 21:5 ratios, I chose the first one because it would give me a more conservative speed, roughly 2.7%. But 10% is indeed a bit excessive and quite annoying even when I'm driving in the city.

    At this point I was pulling my hair, so I phoned Brian at 86tuning. Brian explained that the speedometer in a 20+ year old car isn’t very accurate to begin with (~ 5%), and typically it gives a higher indicated speed. By going with the more conservative 20:5 ratio, I have essentially doubled the safety factor.

    Had I chosen a 21:5 (= 4.2), the indicated speed would be roughly 5% higher than the actual speed, which is more reasonable.

    Another thing to note is, according to www.ae86drivingclub.com.au, the Australian AE86 comes with 22:6 and 19:5, depending on the tire size, and 22:6 being more common. Here in Canada, however, the GTS models having 185/60/R14 tires seem to commonly come with 19:5 speedo gears. For 4.78 final which is 10% higher than 4.3, I would need 21:5 (10% more than 19:5), without taking into account speedometer inaccuracy.

    However, to compensate for my inaccurate speedometer, I would need to change the driven gear to from 20 to 22 teeth, i.e. 22:5 = 4.4!!

    After solving the puzzles, I bought a 22-teeth driven gear (p/n: 33403-19175). First, it’s still available from the dealer. Second, it works with the 5-teeth metal drive gear. Third, I checked against the GPS afterward and confirmed the speedometer to be accurate. Last but not least, thanks Narfy to ordering and installing the speedo gear.

    Lesson learnt:

    You will need 21 or 22 teeth depending on (1) speedometer accuracy, (2) tire size.

    First, before deciding on speedo gears, check how inaccurate your speedometer using a GPS or similar device. For instance, mine is off by 7%.

    The tire size is another factor. The figure in the example (0.580m) is only an average dimension. For your reference, my 195/50/15 tire has a diameter of 0.576m.

    Then using the formula below, you should be able to determine which speedo gear you need.

    *** Calculating speedo gear ratio ***

    With any other final drives, you can calculate the speedo gear ratio that would give you accurate speedometer readings. Let's take the 4.78 final drive as an example:

    Diff = 4.778, or 43/9
    D = diameter of tire, in metre = 0.580 m

    speedo gear ratio
    = (1000 x Diff)/(pie x D x 637)
    = (1000 x (43/9) )/(3.14 x 0.580 x 637)
    = 4.11

    speedo gears = 20/5 = 4.0

    difference = (4.0 - 4.11) / 4.11 = -2.7%

    Therefore, the actual speed will be 2.7% slower than the indicated speed.

    *** Miscellaneous part numbers ***

    These part numbers came from www.ae86drivingclub.com.au.

    Plastic gear
    33403-19135 N=19
    33403-19155 N=20
    33403-19165 N=21 (not specifically a AE86 part)
    33403-19175 N=22

    Metal worm gear
    33481-12030 N=5
    33481-14040 N=6
    Last edited by marvinng; 09-02-2008 at 10:53 PM. Reason: updated information

  2. #2
    Join Date
    Mar 2004
    Posts
    1,341
    Excellent research. This post will be moved to the Tech Reference thread. Thanks Marvin
    A thief broke into my house last night...He started searching for money so I woke up and searched with him

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