Results 1 to 11 of 11

Thread: How does overdrive Annulus effect speedometer

Forum to discuss Austin Healey Sports Cars

  1. #1
    Senior Member
    Join Date
    Jun 2015
    Location
    Albany Georgia
    Posts
    56
    Thanks Thanks Given 
    0
    Thanks Thanks Received 
    0
    Thanked in
    0 Posts

    How does overdrive Annulus effect speedometer

    I figure my speedo reads higher than it should. I have a BN7 and the Eng, trans and OD unit was swapped out at one time or another. 3000 engine, and I believe i have a 100-6 trans and OD. AEC3174 trans, and 28/1447 OD unit.
    So I have an 28/1447 overdrive instead of a 22/3009 and I have the 3.909 rear end. Moss catalog shows different annulus (item 143) and speedometer pinion assemblies ( 168, 169) between those two units. BN2,4,6 is different than the BN7, BT7, BJ7, BJ8. I am figure those parts are what they use to compensate for the different ratio rear ends.

    If so can i swap out those items 143, 168, 169 to get my speedo to read correctly and can I swap those parts without swapping out the whole overdrive. Are they interchangeable???? Will And will any of these 100-6 OD and 3000 OD units fit on any of the 100_6 or 3000 transmissions. I know the output will be different if output is different but are they physically interchangeable.

  2. #2
    Yoda
    Join Date
    Feb 2007
    Location
    California USA
    Posts
    3,884
    Thanks Thanks Given 
    4
    Thanks Thanks Received 
    44
    Thanked in
    41 Posts

    Re: How does overdrive Annulus effect speedometer

    The different annuluses (annuli?) are what's used to achieve engine speed reduction (or output speed increase) between the 22% and 28% ODs. The angle drives are different between the BN2 and later cars (note the price delta). Interesting question, though; I've always assumed the angle drives were 1:1 but, now that I think of it, probably not (speedo cable probably can't handle 3000RPM, so the adapters probably reduce to 2:1 or even more; either way, they are likely the same reduction for all models).

    What you need is a ratio adapter or a different speedometer gearset. There are several places that sell ratio adapters, they will want you to measure how many cable turns it takes to go a certain distance. I got a gearset change when I went to a 3.54 rear-end in my BJ8 from the standard 3.9, but your situation is an odd duck and a ratio adapter is probably the easiest way to go.

  3. #3
    Yoda dklawson's Avatar
    Join Date
    Mar 2005
    Location
    Durham, North Carolina, USA
    Posts
    6,105
    Thanks Thanks Given 
    0
    Thanks Thanks Received 
    42
    Thanked in
    39 Posts

    Re: How does overdrive Annulus effect speedometer

    +1 on the ratio adapter. If you are happy with the current setup and don't plan any additional gearing changes the ratio adapter is a one-time expense. The last time I checked they were typically around $150 plus perhaps the cost of a new speedometer cable (which will be modified by the shop to accommodate the ratio converter).
    Doug L.
    '64 Morris Mini Cooper-S 1275
    '67 Triumph GT6 Mk1

  4. #4
    Senior Member
    Join Date
    Jun 2015
    Location
    Albany Georgia
    Posts
    56
    Thanks Thanks Given 
    0
    Thanks Thanks Received 
    0
    Thanked in
    0 Posts

    Re: How does overdrive Annulus effect speedometer

    And now that I look at the moss drawing, I see the annulus has teeth that mesh with the carrier assy, and those parts are different for different ratios. So I can't change the annulus without getting a different ratio. So looks like with 28% ratio OD, getting the ratio adapter is only way out, I would never that thought of that. Thank you. If you know who sells something compatable with a healey let me know. I will start checking around. Plus my tires are the 185/70 -15 that might have changed things a bit.

  5. #5
    Yoda dklawson's Avatar
    Join Date
    Mar 2005
    Location
    Durham, North Carolina, USA
    Posts
    6,105
    Thanks Thanks Given 
    0
    Thanks Thanks Received 
    42
    Thanked in
    39 Posts

    Re: How does overdrive Annulus effect speedometer

    There are other ways but I assume you want your dash to look traditional. To name a few, there are cell phone apps, stand alone GPS units, GPS portable speedometers that lack an odometer, GPS in-dash speedometers that include an odometer and trip meter, and there are bicycle computers that can be adapted. All will work, some are less expensive than others. However, the ratio adapter is a clean way to keep your original speedometer.
    Doug L.
    '64 Morris Mini Cooper-S 1275
    '67 Triumph GT6 Mk1

  6. #6
    Senior Member
    Join Date
    Jun 2015
    Location
    Albany Georgia
    Posts
    56
    Thanks Thanks Given 
    0
    Thanks Thanks Received 
    0
    Thanked in
    0 Posts

    Re: How does overdrive Annulus effect speedometer

    Yes, I want the dash to look original and see it at a glance.

  7. #7
    Yoda John Turney's Avatar
    Join Date
    May 2002
    Location
    Concord, CA
    Posts
    3,537
    Thanks Thanks Given 
    40
    Thanks Thanks Received 
    28
    Thanked in
    27 Posts

    Re: How does overdrive Annulus effect speedometer

    If you know how many turns your speedometer cable makes per mile, an instrument shop can change the speedometer to match. If you can roll the car forward 52.8 feet while counting the turns of the speedometer cable, and multiply by 100, you will get a good approximation.
    John, BN4

  8. #8
    Senior Member
    Join Date
    Jun 2015
    Location
    Albany Georgia
    Posts
    56
    Thanks Thanks Given 
    0
    Thanks Thanks Received 
    0
    Thanked in
    0 Posts

    Re: How does overdrive Annulus effect speedometer

    They must change a gear in there. That would work for me too. Just got to find a shop that does that.

  9. #9
    Senior Member
    Join Date
    Jun 2015
    Location
    Albany Georgia
    Posts
    56
    Thanks Thanks Given 
    0
    Thanks Thanks Received 
    0
    Thanked in
    0 Posts

    Re: How does overdrive Annulus effect speedometer

    I see the Nisomger people might fix the odo or sell a ratio adapter.

  10. #10
    Yoda
    Join Date
    Feb 2007
    Location
    California USA
    Posts
    3,884
    Thanks Thanks Given 
    4
    Thanks Thanks Received 
    44
    Thanked in
    41 Posts

    Re: How does overdrive Annulus effect speedometer

    Palo Alto Speedometer in Palo Alto (duh) did the gear swap for me (and returned the originals). That corrects the odometer and tripmeter, the speed indicator is a bit of alchemy (which they neglected to do the first time, but a CHP officer was kind enough to point out the error in my velocity).

  11. #11
    Yoda Randy Forbes's Avatar
    Join Date
    Feb 2003
    Location
    SW Florida (Sarasota/Bradenton area)
    Posts
    4,494
    Thanks Thanks Given 
    8
    Thanks Thanks Received 
    35
    Thanked in
    31 Posts

    Re: How does overdrive Annulus effect speedometer

    I just use an app on my cellphone. A glance at it every once in a while is all I need__the speed of a Healey doesn't drift upwards with near the unintentional ease of some of our other cars (that I DO need to near-constantly monitor...).
    http://www.britishcarforum.com/bcf/image.php
    57 Healey BN6L-942 Wine Red/Honey Tan
    99 BMW M Coupe EB w/ ES/TS & 99 M Rdstr BK/BK
    01 BMW M Rdstr Steel Gray Performance Center (factory) Delivery
    16 X5M/DINAN Stg II Mineral White Metallic Wife's 716 hp/702 Lb-ft Turbo Hauler

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •